Mostly for my dogs but I can squeeze through on my truck now, just barely. At 50+ year and 200+ lbs, it is not easy and probably not good for the structure of the truck or my cap. But seeing that they leak, I won't be pursuing that.
It might work for you if you have a good sealing opening on your truck rear window, a good sealing opening on the front of your camper, an long overhang of the camper across the truck-camper gap, and an accordion/inflatable seal between the two.
Just make sure you can reach through the truck rear window to unlock and open the camper opening (most truck caps with an opening front window are unlocked from inside the cap, but you would want to find/make a camper front opening that opens/unlocks from the front).
If it is (please pardon the joke) raining "cats and dogs" while driving down the road, then keep everything sealed, but, on a drier day, you can use the opening as you want, even while driving.
Even if it raining hard, when stopped, you could use the opening I mentioned above and it would likely work well for you, It would also allow your dogs to patrol the truck and the camper while you are sleeping.
