If you are talking a 2-4hp outboard, consider a roll up or inflatable floor model. You aren't going to get on plane anyway, so the advantage of a RIB (and the weight that comes with it) goes away. You can find these in the under 100lb range, so getting it up on the truck cap is likely more viable.
If the cap has a ladder rack, you could probably set up a simple platform that could lean against the back of the truck and then winch up onto the cap rather than lifting. It would take a little bit of fabrication but shouldn't be too hard.
If you are just looking at getting from the campsite to the launch within the campground (ie: topping out at 5-10mph), the wheels that attach the transom are quite viable and wouldn't be hard to make up a set yourself.
