As was said, there have been both, just not many "out of the ordinary"
The most obvious is the Freightliner FRED (Front Engine Diesel) chassis, which was used by quite a number of motorhome manufacturers. This was basically a gasser chassis with a diesel engine. None of the rear engine diesel advantages, just a leaf spring gasser chassis with a really noisy front diesel engine.
Workhorse did a rear engine gas for a while, but they didn't sell enough of them, and then they left the business.
Someone made a MID-engine diesel, I think Newmar used them in some cool air ride toy haulers. Excellent idea, but maintenance was a nightmare.
One really cool gas pusher was FMC, they used a 440 dodge in the rear. Charles Kerault ran one of these, and they are still popular.
Don't forget the GMC's, they used a Toronado drivetrain (front engine front drive 455) to power a motorhome, excellent idea, also still very popular.
But we always come down to cost per unit.
Our current Ford F53's, this chassis is used in about 6 zillion medium duty delivery type trucks. This has let Ford give motorhome manufacturers a really good price.
same with the "standard DP" these are mostly variations of the basic bus chassis. Though most these days are specific to motorhomes, there is still a lot of shared parts and systems, these reduce the end cost of the "normal" chassis.