OH BOY, Most diesel pushers are designed for cocktails for six, dinner for four, and sleep too. There is not a gas Motorhome made that will tow a large stacker trailer...7500 lb hitch max. You could go with a toy hauler pulling a van 4 down (no trailer) is a possibility but 99% are gas. Once you go over about 38 feet you are in diesel pusher territory. A single axle 40’ DP might have 4500 lbs of CCC (amount of stuff you can put in it) and a 42’tag rig will have double that. Agree many DPs only have a 10K hitch... while mine is 10K, a 09 with the same hitch, is rated 15K.
A big DP is going to weigh 45,000+ lbs loaded up and a big stacker will be 15K... 60,000+ lbs putting you into the 500+ HP with 1600+ torque league ...not a Cummins ISL 450 (good engine but only 1250 torque).
Doubt you will find what you’ve specked so modifying one that comes close will be the only option.