A lot depends on your cats. We have been camping with our Bengal cat since he was 10 months old (we got him when he was 6 months old). We have taken him on every camping trip since then. We also take him shopping on a leash and started that a week after we got him. But, he is a very smart cat. For example, he knows that he can't get out of the van without a leash on and knows that he can't be on the dash of our SMB while driving but can when we are stopped:
When camping, we clip a 15' wire to his harness and clip the other end to the van...he loves to get out and wander around and smell new things.
So, my first question would be...do the cats like traveling (or at least put up with it)? Are they fine with being inside all of the time or are they indoor/outdoor cats? If the latter, are they going to want to be outside when you camp?
A poptop is essential. There will be times that the cats will need to be in the van while you are shopping or eating in a restaurant. At those times (unless it is cold outside), we try to park in the shade. If not, we will put up the poptop and open the windows. With the top down (and even with the side windows open), it will quickly get hotter inside than outside. With the top up and the windows open, it will generally be the same temperature inside and outside in the penthouse although it can be warmer down below. We have been fly fishing in the low 80s and will go back and check on him every hour or so but it is always fine. Our cat has never scratched any furniture or the screen door at home so we don't worry about him scratching on the window screens in the penthouse.
You do need a way from your cats to get up to the penthouse (and get down safely). We have an EB-50 and he quickly learned to first jump up on the sofa, then climb onto the counter in front of the sink, then onto the counter over the refrigerator. From there, he stands on his hind legs and puts his front paws on the ledge around the penthouse opening and jumps up. It is then an easy walk along the narrow ledge to the bed area. (Bengel cats do like to be up high and that can be good while camping.)
We too use air mattresses and he knows that he can't come up to the penthouse until the air mattresses have been inflated and the bed made. We haven't had a problem along those lines.
We also take him with us while snow camping and, although we have a heater, it doesn't come on at night. Get gets under the covers with us in the penthouse when it gets cold. Once the heater comes on, he gets down and lays in front of the vent.
We use duffel bags for our bedding, etc. which are stored behind the sofa. We have a blanket for him which we lay folded over the bags and, while driving, he will spend most of the time there, looking out the back door of the van. He knows not to ever touch the driver while the van is in motion but might come up and see us once in a while or sit on my wife's lap. On some long trips without my wife, he would come up occasionally and jump up on the passenger seat and look out the window for a few minutes and swap stories with me before retreating back to his station in the back.
Besides food and water, we have a litter box for him and a cardboard scratcher. As soon as we stop to camp, the litter box is moved to the floor in front of the driver's seat which gives us more room to move around.
(On one trip coming back from Colorado, we camped at a tacky RV park in Winnemucca, NV and I wanted to get an early start home the next day. So, before going to bed, I moved his litter box to the "traveling position". I woke up very early and, after rolling up the bedding, looked down to see him sitting on the floor where his litter box "should" have been, looking very confused! I showed him where it was, he did his business, and everything was fine again.)
I hope this works out for you. We really enjoy having our cat with us while camping...and he likes it too!