Keeping those batteries up to snuff during long term storage

Actually all three methods would work and are currently employed by forum members. Since you have a WFCO onboard, solar would be my choice since 160 watts should suffice for just keeping a pair of batteries topped off. Many do choose to remove batteries and place them on a battery tender type device, we store our rig in our sideyard plugged in and let our Iota IQ4 do it's job.