For over eight years I've been reading posts like this one, and I've come to a conclusion after being on the road for those years also. If you're not comfortable driving a 15 year old car cross country, you will not drive a MH (gas or DP) cross country.
Most of it will be in your head and hands, meaning what you feel comfortable repairing. If you feel comfortable swapping out a fan belt on the side of the road, or if you need to call a tow service.
Remember these things only get driven 2k -10K per year usually. Rust and corrosion will be the problems. For example your hot water heater won't ignite because the connections need cleaning. Your calipers will stick from sitting too long. Service records will show none of this. If buying an older coach and your first trip out has a few issues that you can't fix, bet on the fact the next trip out will be much shorter. If that next trip has issues, bet on the fact it will be a longer time before you take it out again. Doesn't take long before it is a driveway queen.
So the bottom line is would you take a car that is "X" years old cross country, and feel comfortable doing it. Use that same logic when buying a used coach regardless gas or DP.