If you're lucky enough to be able to park it inside, they make a nice battery trickle charger. Called a "Battery tender" or similar name, and they go for about $30-40. Plug it in, and it tops of the battery with a slow charge as needed. However, not sure how it would work with two batteries.
If you're outside, they also sell a decent solar charger that does the same thing. Just run the cord and put it on the dash or somewhere exposed to sun.
Otherwise, you may just have somebody fire up the truck and drive it around the neighborhood for a half hour. I'm sure some TDR members would be willing to volunteer some seat time. ;-)
TP