The one thing that will run down the batteries in no time is a 3-way fridge running on 12V at 120-200 watts. Can you exclude that as a reason?
First, unplug camper to eliminate any cause from that side.
What voltage do the truck batteries read?
What voltage does the camper battery read?
How old are the batteries?
Do they accept a a charge? Either from an external charger or from the truck alternator after a jump start?
Possibilities:
Truck battery died
Lights or other electrical device left on
Excessive load from camper, see beginning of my post (but that doesn't seem to rhyme well with your story)
Not a professional here, better opinions welcome.
