the cummings 94-98 is th most powerful of the 12 valve non electronic engines It would be a good combo but a job to put in. Might be pretty loud inside you probably will have to do something with rear gearing
I have a 76 18 ft m/h with a stock 454 400 combo i get about 10 mpg when hauling my 18 ft trailer and car 55 mph max. can always want more power but works ok. they are a brick with wheels

Heres a suggestion keep it simple pull the 350 stroke it as big as you can go , vortec heads good towing torque cam headers etc . maybe an extra 100 hp out of your original if your lucky dont have to change any gears or anything else keep it cool and run good oil and it should go a long time. a cummings or built big block it wont be but it should do the job. My preference would be a hd 5 spd manual but not practical in a m/h.
just my 2 cents and suggestion