Advice please!

Open in Nov - All 3 (KY, KB, KK)
Accessible by car - KY is most accessible by car. Having been in KK this closed season already, I can share that the road is now quite rough and there have been a lot of landslides, esp during rain of last few of weeks. Getting to Panontheung was NOT possible unless you have a pick-up at least. Indeed just a couple of weeks ago you needed a 4x4 style car (i.e. high clearance) just to get through one of the streams before km 18. The concrete ford over the stream in Bang Krang Camp had also disappeared one night in a storm.
Want to see Gibbons and Elephants - KY and KB
Night drive - KY

I would think KY suits your needs best but there are others on the forum who have a much better knowledge of KY than me and I sure they can help you more.

I would guess that Khao Yai is your best bet. My homebase, ;).
I haven't been to Kui Buri, and hear that's the best place to see wild elephants, people say it's pretty much guaranteed. Khao Yai you need more luck, but still if you keep trying...

Gibbons are not too difficult, though I guess if you don't know the area it can be hard. Depending on how much they're calling.
From the first viewpoint till the visitor centre (Pak Chong side, which is the better side for gibbons) your chances are good. Behind the parking of the first viewpoint you might be lucky to spot a mixed family of both White-handed and Pileated gibbon, but they don't show up too often, you might hear them. When you continue you notice better quality forest around the road and around the cobra crossing sign you need to start looking up. Another good area where they often are close to the road is around the first parking area, which they call Dong Krating (no sign though; few hundred meters past the elephant croosing sign). There is an unofficial trail starting on the side of the car park usually a gibbon family hangs around there.
Next good spot is the km33 trail. You could follow the official trail, or even better explore the many trails around there but very easily you'll loose orientation. Certainly don't try without compass and preferably GPS. Most guided day tours spend there time here, for a good reason. Several family groups live around there, and there are some beautiful large fig trees. There are more good trails.
Pileated Gibbon, try the road to the Pha Dieow Die viewpoint, both before, around, and just after the grassland; listen for the bubble/ trilling calls which are Pileated Gibbons. After 15:00 it will be hard to find gibbons, but that's when you can switch to elephant searching.

The late afternoons tend to be best, though I've seen them appearing at any time of day, even the 'hottest' hours. Seeing an elephant while hiking is rare. So drive the road that leads to Haew Narok. You'll see the tour companies do the same after 16:00, and again for a good reason. Especially the area around the two salt licks is interesting.
Basically you can be lucky all around the park. The last 3 or 4 days a herd of 9 has been hangin around the two saltlicks near the road close to the Nong Pak Chi watchtower parking area.
At the watchtower itself I haven't been too lucky, I've seen them there but I think the Southern road is best. Anyway they could come out anywhere anytime. You could try to chat a bit with some rangers if they have seen any recently or even that day. And if you're interested you can ask them if there have been any sambar kills by Dholes recently. It's worth checking.

If you go with a local guide you're pretty much guaranteed to find gibbon and increase chances to see elephant as they are in touch with other guides and park rangers and usually hear from each other as soon as an elephant has come out to any accessible area. However finding it on your own, is more fun, at least that's my opinion.

The nightsafari can be great, even though you're rarely the only one on the road. Take the 500baht private option. Don't get tempted by the 50 baht large trucks.
Good chance to see Small Indian Civet, Asian Palm Civet, Malayan Porcupine, lots of deer, and possibly more like Slow Loris, Buffy Fish Owl, Small-toothed Palm Civet, Dholes, Elephants, Pythons. You need luck to get a driver that is willing to take the time, and hopefully the person operating the spotlight is not too lazy and knows where to look. Though, in general if you ask, they are happy to let you operate the spotlight yourself.
Even though it's not perfect I think it's still fun to do. Some people believe you bother the animals too much with spotlighting, but I've seen the same specimens return day after day to the same areas, no matter how many night safari trucks have passed, and I don't believe it does much harm, certainly not more than you do by entering the forest during daytime.

Enjoy your stay, no matter which park you choose. Don't expect too much, it's not easy to spot large mammals. Even harder when you don't have much experience/ don't know the area.

If you have more questions, just ask...

Starting at 6:00AM can be great especially because there will be little other traffic.
It's definitely good for birding, but for most mammals (certainly for gibbons & elephants) it seems like chances are good throughout the day.
The forest cover and elevation in Khao Yai prevents it from getting hot during the day, so unlike the open savannas in e.g. Africa, animals are less restricted to very early and or late afternoon.

I'm never good at waking up early, even though the mornings are beautiful in these parks. I should learn to get to bed earlier and wake up with the sun... ;)

It's best to travel to Pak Chong if you travel by public transport. At Rimtarninn hotel in Pak Chong town is a car rental branche. From Rimtarninn you need to follow the road a little back in the direction of Bangkok and then follow the sign Khao Yai to turn onto Thanarad road (2090).Just keep following this road and you'll get to the park gate (30min drive).

I would recommend to look for a hotel near the park, or camp (the park rens out all neccessary gear expect a total of 350 - 400 baht for tent, sleeping bag, mat, and pillow + camping fee)/ take a cabin (if available) inside the park. It's hard to find decent budget options near the park. The cheaper guesthouses are a bit closer to Pak Chong town and therefore further away from the park.

The visitor/ information centre is 14km from the park entrance. There you can ask for a park ranger though most don't speak English; they know the trails and some might do their best to spot wildlife. In my opinion most local guides of local tour businesses are better at spotting animals and most speak good English, but that would require booking a tour with any of the companies in the area easily found on google or in guide books.
If I travel to a new area I prefer to go on my own, even though I'm well aware that I would see more with a good local guide. Spotting the animals yourself is more fun in my opinion, but it depends a bit if you have good eyes to actually discover something.

Good luck!

Don't forget to tell us what you see John. Good luck!

Fantastic!