Thanks to all of you above for all your knowledge and good advice. I checked the date on my batteries and, believe it or not, they are dated 2010! I'm pretty sure they were installed by my PC's first owners who purchased the PC new from the factory and owned it until 2018. Do you think it might be time to replace them, lol?!?! Pretty good run for the two 12V wet cells I'd say! May they rest in peace. I'll replace them with some 12V AGM batteries this week. (I'm aware there are better batteries and methods (e.g., two 6 Volts) but time before we hit the road is tight and we are on a strict budget right now) which, given our needs and our 100W solar charger, ought to suffice for a while. Though I still have some concerns about parasitic drain, I'll pursue that if it appears to still be an issue with the new batteries.
The older solenoid style only drew current when "on" which was/is another reason to turn the 12v off when in storage. I don't know the particular solenoid used by PC but a typical solenoid of that type uses about 10 to 15 watts for the coil to engage the contacts or about 1 amp.Since the coach battery(s) are typically rated about 200 amp hours so 6 to 8 days without any charge before they are totally discharged which is always a bad thing.