yes, safe bet that your radio, or something is pulling a parasite load.
One of the first things I did when i bought mine was to order two cutoff switches and install them. When i store my unit, i disconnect both batteries completely. It's a tiny bit of a pain in the neck, but only takes a minute to store and a minute to put back into service.
I ordered these from amazon.com
BS200 Top Post Battery Master Disconnect Switch
just under a year so far..... and so far so good....