I just plug my motorcycle (dresser) up about once a month. You have to remember the clocks running on these vehicles while they set uses current as well. Battery woes comes from freezing as well. Ideally take them in the house and charge them monthly. My Harley battery last for about 5 years, original battery in my 07.last spring when I got home to CdA from Reno/Sparks my battery was also dry ... used a tender all winter while I was down here
so I'll be checking 4 different rigs again when I get home , [4 tenders in use now] .... I didn't think it was a tender issue as I had my rig on it's side once , but maybe I have one tender malfunctioning [or more] ... using the Deltran brand tenders.
I added additional tenders this fall, to cover all my battery rigs left for the winter ... couple with very pricey Odyssey batteries now hope I didn't cause some major loss $$$$
You can find them numerous places, this is like mine, for about 25-30 bucks.