A few thoughts to further muddy the soup. First, very sorry about the loss of your wife.
Many manufacturers out there that make both smaller and large slide in campers and to me, it seems that most of the smaller campers are designed to take advantage of very the popular short box, "crew/extra/supercab, etc" format of trucks, such as yours. Obviously, this category is popular because of both the shorter wheel bases (generally) and the ability to carry multiple passengers, for example, families. I am most familiar with Canadian companies that manufacture mostly fiberglass moulded campers. Today, none of these companies manufacture a camper that is designed to go flush into a pickup truck box, be it 6.5 or an 8 ft. And to boot, none of these campers are designed to be used (can't be) with the tailgate, down or otherwise. Some of these are pushing 3' of overhang in a 6'5" bed, often with double propane tanks and/or black/gray water tanks out there in the overhang. Although very strong, the fiberglass is also heavy. I just can't see your foot and a half overhang with modest weight, is going to be a problem. I had both a Six Pac and a FWC campers with boxed plywood on the bottom and used them without the tailgate, with nary a problem. If removed, the tailgate is an easy 70 lbs to subtract from your calculations.
On the forced air propane heater, they are simple, easy to trouble shoot and fix. If you already have the propane system(s) in place, I would question the need to add both the hardware and an addition fuel to what I think you want to be a fairly simple set up. JMO.
The solid foam insulation as a camper/bed interface is a great way to go. You can buy the exact thickness (within 1/2") that you need, it obviously gives you extra insulation, it is super light and it's compression resistance per sq. inch is waaaay greater than any weight your camper is ever going exert on it. Up here, it comes in 2x8 foot sections, buy two of them, chop them off at the end of the box, paint the ends black so they don't look too dorkey, use the left over pieces as the buffer between the front of the box and the camper and then forget about it.
Last thought from an old guy, go with the camper, you wouldn't be sorry