Usually most RV parks even ones with the 10 year rule go more by appearance that the actual age. the rule just gives them a reason to keep a junky looking RV out. This is not always the case but I have had several older motor homes and have never been turned away. I keep them looking as close to new as possible. My current rig is a 2001 Winnebago Brave that I have had several years. It has around 30k miles on it and I have never had any appliance problems but I have had 2 problems with the chassis, one was an in tank fuel pump $$$$ and one was a brake issue. As far as the AC going down the road, in moderate weather the dash AC will keep the house cool enough for most animals but if it gets too hot it is not a big deal to run the generator and the roof air. I think you really should either have a tow behind vehicle with a MH in case of break downs so you have a place for the pets or it sounds like a trailer with a van or big SUV would be the best for you and the pets.
A class A or Class C in the mountains is not a big deal but towing a heavy vehicle behind it in the mountains may be, My MH does not like towing my Jeep over a big steep grade, the engine is OK but the transmission heats up!
