Chevy 454 and Ford 460 are both well proven engine cores, it is not a mistake to go with an engine built on either one.
If the problem is only a blown head gasket (rare in modern engines) the $18,000 estimate means "we don't want to do the job." You are talking about an under $100 part (and at most another few hundred dollars worth of gaskets, hoses and fasteners for everything that needs to be removed and replaced) and a couple of days worth of labor. Labor depends on engine access.
However, if the blown head gasket means more serious engine damage, a remove, rebuild and replace for a big-block V8 in a Class A gas motorhome should still be a lot less than $18,000. But it could still be a lot more than your motorhome is worth.