Yes, it sounds like the ABS on your truck. When towing the trailer's momentum is still pushing forward (even if the trailer brakes are locked down). The ABS is reacting now, like it would if you were on Ice or a slick road. They react and put more resistance on the wheel that will correct a skid (so it thinks). Your ABS thinks your in a skid and attempting to correct, but it's not really on ice or wet roads, it's the push of the camper from behind. But the ABS does not know this, it's just sensing a skid in process.
I don't think there is any way you can test this theory, even if you set your trailer brake controller to lock-up the brakes, the momentum of the camper will still push forward.
Switching campers and switching tow vehicles is a very good idea. It will determine immediately if the problem is with camper, or truck. I think you'll find it's the truck.
Edit: in a nutshell, your truck to too small for the camper. It can't stop it.
Edit Edit: I re-read your post again, and still think this theory is right. When you towed your 5er, the 5er was much heavier. When you applied the brakes on the trailer, the sheer weight of the camper pressing the tires into the road caused enough friction to counter act the forward momentum of the truck. The lighter camper does not have the same friction on the tires against the road. Being lighter, the brakes will not engage as strong, allowing for that forward momentum, thus causing ABS to think its in a skid. Does this make sense?
