Great advice, too bad it is near impossible to do for a number of reasons. One of the first is everyone is different and has different expectations and what you think you want is often different from what you learn to want and need.
One of the first considerations and often one of the first to change after using the motorhome is how big should the motorhome be. Long can give more living space but can restrict where you can park especially if you like to use State and National Parks. As an example one of the Oregon State Parks we like to use has slightly over 100 full hookup sites for 34' or less motorhomes, Thirty (approximate number) for homes < 36', 12 for units <38', and 2 for 40 footers.
That would seem to favor 34' or less but most class A motorhomes less than 34' are difficult to drive because they do not track well. A 34' home doesn't do a great job when compared to 36'. Thirty-eight to forty foot is the drive-ability sweet spot.
Next on how big is what you get in general. A 34' motorhome will usually have front seats that turn around (some 180 degrees, some less), a dinette for four, a couch that makes into a double bed, an okay kitchen, a bath and a queen size bed with some storage. A 36 footer will have everything the 34 footer has plus a chair/recliner in the living area. Thirty-eight feet will add another chair/recliner, slightly more kitchen and bath space and more storage. Forty feet and you get more storage and likely a makeup area in the bed room, over 40' and you may have a 1/2 bath as well as a full bath behind the bedroom.
Of course next to size the most important decisions are diesel/gas and Class A or C. The A or C decision is mostly what floats your boat. Engine in back has less engine noise than one up front. Diesel/gas has two main factors, price and systems. A diesel chassis is designed for a service life of 700,000 to 1,000,000 miles, gas chassis no so much. The price difference can be offset somewhat by buying an older diesel motorhome. The other advantage of Diesel pushers is even entry level homes have
complete and robust systems vs. most of the time with gas power the systems
are not as good or complete.
While age of a used motorhome can be a factor, mileage on a diesel pusher is not much of a factor as you seldom find one with more than 150,000 miles, about 15% of the bus service life. Pre-2000 motorhomes likely have dated systems, worn interiors and brittle plastic. Newer units have better digital controls but newer than 2006 the emissions control electronics can cause major problems. BTW, my preference if I were buying my second motorhome would be a 2006 40' top of the line bus like a ForeTravel or Country Coach.
More to follow,
ken