I am a full time RVer and currently have a 2007 Winnebago Journey 39 ft diesel pusher (Cat 350). My pevious RV was a 2003 35 ft Winnebago Adventurer (gas, Workhorse chassis, GM 8.1 litre V-8) so I can compare the two. We tow a car and towed the same car with both RVs.
The DP is much bigger and has a greater carrying capacity, which for fulltime living is a blessing. We travel about 15,000 miles a year and currently have 51,000 miles on it.
The gas Adventurer worked very well for us before we traveled fulltime. Over five years we put 45,000 miles on it.
To compare these two units, the diesel is much more expensive to maintain, not only for the engine but there are lots of items to be serviced, transmissions, differential, air brakes, filters, than on the gas unit. The gas unit is much, much cheaper to maintain.
Both RVs got about the same mileage, 7-8 mpg. Power was about the same. The ride on the DP is certainly better with the air suspension over the gas chassis leaf springs.
If I was looking now to buy another RV it would certainly depend on what I was going to do with it. If it was for occasional weekend and several vactions a year type use, I'd buy the gas MH. However, for fulltime, I'd go with the DP for its bigger carrying capacity, larger size.
Just my view, hope it is of some value.
Best Regards!