If there's no load on them you really only need to keep up with the self-discharge rate. Solar panels should do it. If you're parked in a garage with power a 120V "battery tender" (aka trickle charger) would be ideal. Sometimes if the garage has an overhead incandescent light you can cheat and use an adapter that screws into the light fixture and adds two electrical sockets.
Depending on what converter you have, keeping the van plugged in all the time might even overcharge the batteries and shorten their life. Mine seems to run at 14.3V, which is far too high for a long-term 'float' charge.
Mostly you want to avoid having them sit there in a fully discharged condition, which can sulfate the plates and even result in the battery freezing. BrianW's suggestion is a good one. When I had to store a van for the winter in a place with no power, I took the battery with me and kept it on a battery tender at home.
If none of these options work for you, disconnect the cables (to avoid parasitic loads) and they'll probably retain most of their charge through the winter. You're taking a risk, but worse case you need to replace the batteries sooner.
1990 E-250 Sportsmobile w/ penthouse top, converted when new by SMB Texas.