Welcome to the forum! Lots of good info out there, and also lots of opinions, like the diesel vrs gas you brought up. Lots of answers, and lots of good input, but no wrong answers! Depends on the buyer and his/her needs and desires.
There are also lots of used rigs out there. To answer one of your questions, no, the dealers cannot just re-designate the model year, that is in the original MSO or Manufacturers Statement of Origin placed on the coach by the factory. That being said, the coach receives the MSO by the factory AFTER the coach is built onto the chassis. Chassis may come from Freightliner or Spartan if diesel (some other companies make their diesel chassis from scratch, but the majority are either FL or SP).
If looking at a newer gasser, the only choice is a Ford Chassis. Chassis may be a year or two older build than the actual date on the MSO.
If a used one, depending on year GM made different chassis using the Workhorse engine, but stopped I believe around 2007, but others can point you to the exact date.
Whatever you purchase, if used especially, regardless of whom you purchase from, private party or Dealership, GET THE COACH INSPECTED, NOT by the Stealership either. Have BOTH a chassis inspection, AND a coach inspection, probably two different inspectors. Will cost from $150 and UP for each inspection, probably in the $350 to $700 range for the Coach inspection from a reputable inspector. Have the purchase contract contingent upon satisfactory completion of BOTH inspections. If the sellers balk, walk away, there are other coaches out there!
New OR used, pay particular attention to date codes on tires. Also figure in replacement cost of all batteries if not being actively and regularly used. Figure $2,500 to $3,500 for tires, $600 to $1,000 for batteries if equipped with two chassis and 4 or more Coach batteries.
Good luck in your search, we upgraded from a travel trailer to our current coach and couldn't be happier, immaculate condition, brand new tires, paint job, one owner, meticulous maint. records, and LOTS of extras, for under 50K. They're out there, just gotta look.