There's two things that come into play here. One is the engine and the other is the chassis. Diesel engines are more efficient than their gasoline counterparts. You'll get more torque, and it will be at a lower, more usable RPM, than the gasoline powered engine. You'll also get better fuel economy as long as you are talking apples to apples as far as the coach goes. And, yes, they do cost more.
Most RVers think of a diesel pusher when they think of a diesel motorhome. With a DP you get a whole new level of chassis underneath you. Air ride suspension versus leaf springs, raised rail frames that give you a big pass-through basement storage area, much more cargo carrying capacity, and the quiet of a rear engine, amongst other benefits. But, again, this doesn't come free so you pay substantially more for it.
If all you are looking for is the torque and economy of a diesel engine, various manufacturers are using the FRED chassis from Freightliner. FRED is a front engine diesel chassis that's pretty much the same as a gasser chassis except it has a 300 HP Cummins ISB up front. It's refined really well so you gain some of the benefits of a diesel rig, most notably torque and great fuel economy, with only a slight upcharge over a gas coach. And the noise level is surprisingly low for a front engined diesel. Tiffin has had these in the Allegro Bay for a while now and have now put the FRED chassis under the entry level Allegro, which makes for a very affordable package - both up front and in extended operating costs.
It all depends upon what you are looking for in an RV. There's a place for everything and I'd suggest you do some tire kicking but be sure to test drive the various chassis so that you can see what best appeals to you (and your checkbook