When we had our Northstar pop-up TC I installed a self-contained motion detector alarm I got at Home Depot, IIRC. Had a keyring remote, so we would shut and lock camper door, then just point the remote thru the rear window to activate the alarm (a little LED would blink to let you know it was set). The times I would forget to de-activate it prior to opening the door gave you about 5 seconds to hit the remote or it would certainly scare the @#$% out of you! Had it mounted just above the pass-thru window so it was pretty much restricted to the rear door. Walking past the clear glass window didn't set it off, just opening the door. To the best of my knowledge we never needed it, or if we did then it did its job well, since we never lost anything from inside the camper! ST