Tough call and onlyyu can make it.
Most diesels are built of sturdy chassis and have quality air suspensions, though there have been a number of lower priced models on leaf spring chassis in the past. And some of those had wimpy engines and mediocre transmissionsas well.
Late model gas chassis are excellent and have done away with the weakneses of their earlier brethren.A Workhorse W-series chassis (2001 and later) or a Ford V10 chassis is a very capable platform up to about 20-22,000 lbs of eicle weight 926,000 lbs combined weight). If that's enough for your rig and trailers, consider gas. If not, go with diesel.
I would pick an older, high-line rig over a newer entry level one everytime, though. I thnk you get more value for the dollar in a used high end motorhome.