Trickle Charger for Ford Diesel

I have a 1999 Ford F250 diesel to carry my TC on. During the winter, I usually take the camper off the truck and store the truck in my garage. Works great, except for the battery. I am looking for some advise on trickle or battery maintenance chargers to keep my truck batteries charged during the winter. The camper is fine as it is maintained with solar.Since there are two batteries in the ford diesel I would like to know what others do to keep the batteries charged. I don't want to remove them from the truck, mainly because of my age.

Thanks,

Jim