Diesel vs Gas
First, my background is one who has owned an RV since 1972 and we have been fulltime 4 1/2 years. And we chose to do so in a gas chassis, slide free, 35' motorhome.
I strongly believe that the first thing to set is the budget for this purchase. Those of us who are retired must keep in mind that while our expenses will drop with retirement, the normal costs of life will continue to rise over the years ahead, while our incomes probably will not. So a comfortable cost is vital to happy retirement. You seem to have already chosen a motorhome, but keep in mind that there are advantages to other RVs as well. Studies show that in fulltimers, slightly more than half live in motorhomes with no seperation between class A & C. Probably 3/4 or the motorhomes are class A. The fifth wheel homes make up about 40% of the fulltime community and the remaining 9% is split among all others.
A fifth wheel and tow truck average slightly less, but if the owner actually goes to a truck that is rated to two the weight of the trailer, that price would approach the cost of a motorhome. So at least look closely at the fifth wheel selection as they do have more space when parked than do even class A motorhomes.
The greatese advantage to the class A is ease of set-up and the amount of outside storage. Generally, far more time is spent living in any RV, sitting still, than on the road once you are retired. So livability is very important. And sitting still, there is very little difference between the diesel chassis motorhome and the gas powered one. But there are differences when traveling that must be balenced aginst the price difference. On a new class A motorhome, the cost of a diesel chassis over that of gasoline ranges from $25K to $35K, depending upon the type chassis. There are several, raised rail, standard frame, and variations of unibody type of design. All of these cause variations in price and value. For a gas chassis, there are only two, Ford and GM up to the year 2000 and Workhorse since 2000 when GM sold out their motorhome chassis division.
As a rule, diesel chassis will have a larger cargo capacity than do gas chassis, but not always in the lower priced ones. Recently gas chassis have increased in gross weight and some now have excellent cargo capacity. Realize also that the Workhorse chassis weighs about 1500# more than Ford's and thus the same RV will have about that much more/less cargo capacity, depending upon which you choose. In diesels the range is much greater. Gas chassis range in gross weights from 18K to 23K for class A's, while diesel gross weights start at about 23K and go up to near 60K at the high end bus conversions.
As a rule, the gass chassis is less expensive to buy and to maintain, but it will last about 150K miles before major mechanical work (with proper care, use & maintenance) while a diesel will run from 150K to 500K for the same service. The smaller diesel engines are not those from the bus industry, but from the diesel pick-ups so they don't have the same life term. Also, currently gas usually costs a bit less than does diesel, but diesels usually have at least somewhat better mileage. The speed of travel plays a major part in mileage and if you drive fast, the advantage goes more and more to the diesels. At 55mph there is less difference than at 75mph. Diesels have less maintenance, but it costs more as parts and labor will always be more expensive on a diesle. But most diesels have air ride and brakes which are superior to springs and hydralic brakes of a gas chassis.
Slides are another issue. As a general rule, the motorhomes with no slides cost less, and have somewhat less maintainence. But the slide does make for more living space when sitting still. While traveling, slides do interfer with interior space, so be sure you study the rig with slides in and out, since slides will always win if you do not consider the use of your motorhome when you stop for lunch or wish to spend a night in a tight parking lot. Also, if shopping used, the slide free unit will cost less, just as it did when new and while those who advocate slides always say they are trouble free, if you read all of the RV forums you will find frequent threads about problems with leaks and mechanical problems. No feature of an RV is completely trouble free. So the question is if it is worth the extra maintenance and repair?
There is a lot more to the choice, but I have rambeled long enough. Let's give someone else a shot too!