Lots of good advice here. Of course I'm biased because I have a heavily-optioned 2009 Hawk listed here on the forum for sale. Nevertheless, I agree that, given how much a new FWC costs, a used camper in excellent shape is probably worth around what the owner has invested in it.
On the other hand, truck values (as others have said) can be had easily from comparable sales. Lifts and other aftermarket products aren't worth that much on the used truck market and may actually be a detriment because of the cost to remove them to return the truck to stock or near-stock configuration for the intended purpose. And, considering radical changes like 6-inch lifts may actually have caused damaged to the rig, one needs to be very careful.
A good comparison is the truck that the owner paid $2000 for a beautiful graphic of his girlfriend on one side of the bed and another of his dog on the passenger side. You as a buyer don't want either so the truck value is actually less than normal because of the expense involved in removing the graphics. If your buddy doesn't understand that, then you'd better shop elsewhere because friendships are more valuable that hardware. In my opinion, of course.
Good luck.
Ed