Are you parked near 110vac outlet?
On ours I ran connector leads for a battery tender from the coach batteries to the engine compartment, then another set of leads from the chassis battery.
During these cold long term storage times, I tuck the extension cord under the hood, and with a multiplier plug I connect two battery tenders connected to each lead during my winterizing period.
During summer months, when not traveling, I have an RV outlet at my home parking spot, this way I leave the a/c & fridge on so I don't cook the inside of the rig with the power cord to RV plugged in, this way the converter maintains the batteries.
I don't worry too much if there's a parasitic draw.
It could be the CO2 alarms, or radio, but I think they are de-powered when use switch is turned off.
Just a consideration for you to ease the worry....
Meanwhile, a car port roof is on order, 40 foot long x 13 foot tall, in case I up grade from this 30 footer!