For the tailgate I have only used some basic 3/8 thick EPDM weather seal on the vertical surfaces, but it doesn’t transition well towards the top. For the front corners, I used all purpose silicone caulk (clear hides better) and some backer rods. Pictures of the front corners are probably hard to get and really show much. If the camper has to be taken off, I would probably just strip/remove most of it and redo it. Don’t imagine that happening too often, at least for me.
Weatherstripe (very similar but might have been slightly different dimensions): https://www.homedepot.com/p/M-D-Building-Products-19-32-in-x-10-ft-EPDM-Cellular-Rubber-Auto-and-Marine-Weatherstrip-01033/100175299
And actually, I think I saw @d.shaw post up those items for the tailgate on another thread, and I was going to go down that road too to improve my previous efforts…I have a long way to go to really get the sealing where I want it. (I almost titled this tread “bed stiffeners and sealing”, and my OP definitely is covering both 