Although some options appear from time to time with smaller front engine diesel chassis, and gassers are pushing toward 26,000 pounds where DPs start, what it really boils down to is HOW BIG.
A motorhome fitting a chassis 24,000 pounds down to about 14,000 GVWR is gong to be gas. 26,000 pounds on up, it is diesel. 6,000 to 12,000 pounds, you might have to make a choice, but that is mostly outside the type A market in North America. Diesel dominates under 8000 pounds in most of the rest of the world, whether the truck is hauling a house or a load of bread or flowers.