When your camper is "turned on" there is drain from stuff like CO and Propane detectors. Maybe its only 1 amp but that will drain a 100aH battery in 100 hours. That is only 4 days and your battery is 100% dead (your not supposed to go below 50% charged)
I don't know if your truck wire to camper is live all the time or not, I do know Ram stays hot and yes it will draw down the truck batteries if that line stays hot.(when camping, I use to always unplug that line as a safety feature so I was 100% sure the truck would start when it was time to go home).
On our camper we added Solar charging system because we camp in the National Parks at beach with no electric, but a side affect is that kept the batteries charged in both camper and truck.
I now have a new RV but I have to store it in a field that has no electric. This new RV has a 100w panel on the roof (have not added more power yet but that is in future).
I store under a canape which prevents my builtin system from working, so I put a 100w panel on a cargo trailer that sits on my storage site behind my camper and is not covered.
I wired 10G back to the rear bumper of the camper so I can plug this extra panel into the RV system when in Storage. (I can also plug this trailer in while camping)
That 100W keeps my Coach charged (Two 27 series AGM) AND my chassis (one 30 series AGM)all the time.
since I installed my Panel so I could tilt it, others see it and now I see many other panels popping up in the storage field. Most appear to be 50w which might be enough but I do know my 100W works for me.