As long as the engine compartment has reasonable ventilation, I do not believe that headers will result in any heat problems. The OEM manifolds are known to crack and leak, but normally headers is the solution, not the problem. Headers will run tons cooler than manifolds ever did, as long as you have free flowing exhaust to let the heat out. My old 454 ran so much cooler once headers were installed, and the Magnaflows.
The biggest problem with the P30 chassis is that they are often overloaded, the length is stretched. If the coach is 30 feet or less, you would not have any inherent handling issues. Longer lengths can be improved, but have their limitations. HMC is typically a big tall coach. You will be limited in what you can do to improve it.
If you've not made the purchase, and like the idea of a front wheel drive coach, you could look for a late 80s Revcon. I have no doubt DW would like the interior, and if as long as the suspension is in properly maintained condition, you can't beat the handling. When I drive out west, it is not uncommon to set the cruise at 80 and just roll.