Have you considered a higher end, used fifth wheel? DRV (formerly Doubletree) makes a residential quality unit that is expensive when new, but like all RV equipment it depreciates rapidly. You might find a decent used unit priced within or near your budget.
DRV's have 3.5" thick insulated walls, even in the slide outs. Most have thermopane windows, and they boast insulation values that rival most stick built homes.
It might save you money in the long run to spend a little more up front on that kind of construction, especially if you are facing paying your own electric bill in a park every month.
Good luck.