Oxyacetylene and lots of it. Remove as much sheetmetal and the dog box via bolts and ratchet. Then prepare to start cutting. The more you can cut the easier it will be. Downside is that having it hauled off for scrap becomes a problem.

That's why there are so few motorhomes left.

If it is a 413 industrial between the peanut sparkplug heads and the low compression, it isn't worth doing. That's what I mostly remember seeing in these larger motorhomes.

Otherwise it will be a 440 with cast crank and fat dampner. IMO well worth the trouble.

Once they're bolted in there though, it is an almost impossible task to remove them the way they went in. Almost all of them I've had over the years, have been cut out. Sorry for no better prognosis.