Buy yourself a good condition used Phoenix Cruiser model 2350. We bought ours new in 2007 and we love it to this very day. CLICK HERE TO SEE MANY PICTURES OF OUR 2350
. It's great for the two of us, but would be even better for a single older fellow like yourself. It's easy to drive because of the narrow 93" width, low 10 foot height (to top of a/c unit) and it's great aerodynamics. Our 2007 2350 measures 23'-8" end-to-end. They are also better made than most other brands. Finding one stored indoors throughout it's life would be exceptional. Because they are smaller, many are stored indoors. Phoenix Cruisers are fully featured with decent capacities like the standard class Cs have. It's just a little more compact. Ideally I would not go older than a 2007 to get two house batteries and the whole house inverter. Prior years had one battery and no inverter. If that works for you, then limit yourself to a 2004-2006 to get the integrated spare tire in the fancy rear wall section. The entry door is also better starting mid-year 2004.
Dealer or private party, it matters little. I personally would stay with the ever popular gasoline engine, either the 5.4L-V8 or the most popular 6.8L-V10 in the E350 or E450 chassis. The older 6.0L diesels in those chassis had serious issues that never got resolved.