Try a Chevy 454 and a 4L80 transmission. The 35 foot motorhome I inherited from my parents has that setup and it's been all over the USA, including traveling the ALCAN highway to Alaska, with no engine or transmission concerns. I don't know what the gross weight is, but it handles the boxy motorhome at highway speed with no loss of performance. Gas mileage sucks, but that's to be expected when shoving an overgrown packing crate down the road as 70+ MPH!

The 4L80E transmission needs a computer, but so does the fuel-injected 454 that mates with it. There are aftermarket electronic controls available, but they're pretty spendy.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!