For the vast majority of motorhomes, the relative longevity of the engine is irrelevant. Other parts (such as house stuff) generally wear out well before the drivetrain is worn out. The current Ford V10 engine and transmissions are quite reliable and long-lived if given proper basic maintenance.
For class A motorhomes, a diesel engine nearly always also implies a rear engine chassis, usually with air suspension, while a gas engine would be in a front engine chassis with leaf spring suspension. There are definite advantages to the former in terms of comfort when traveling, which technically have nothing whatsoever to do with the engine technology but still come along with it. (In other words, there's no inherent reason why one couldn't construct a rear engine gas powered motorhome with air suspension, or a front engine diesel unit, but they are both largely nonexistent.)