I just purchased my first diesel pusher, and Allegro Red (rear engine diesel)38ft with a cummins 340. My previous 35+ years of RVing have been in gas coaches, a 454, and Ford V10, and a Workhorse 8.1, respectively. My current experience is with only 1000 miles on the diesel. WOW, how nice it is. The ride is infinitely smoother on California's rough neglected freeways. It is quiet, my wife and I can have conversations while driving, and I can actually hear the radio! even going up steep grades. Going up the Grapevine, a really long 6 to 8% grade I had no problem maintaining 45 to 50mph with the same quiet soft ride. Truthfully, I could gain more speed with the gas 8.1, but I would rev at 4k rpms and the noise level was pretty rough. Going down the Grapevine was a total pleasure. With the engine brake engaged I went down the 8 mile straight grade at 45mph without ever touching my brake. With the gas RVs I would constantly tap the brakes, and at the bottom I would pull out at the first town (at the bottom of the grade) and let the brakes cool while we took a little personal break.
In conclusion: If budget is not in the mix, diesel is the way to go. Of course diesel is more expensive for most people, in base cost, in maintenance costs, and any saving in fuel probably would not apply for the low mileage most RVers put on their coaches.
JMHO