I'll just tell you what I did and you can think about it...
First, I bought a NATO slave plug so that I could plug the truck in without messing with the seat. They can be pricey but a careful watch on the evil auction site turns them up for reasonable prices.
Second, I bought a 24v battery tender. I bought a NOCO Genius smart tender. It's got electronics to prevent overcharging. It's 7.4 amps, so it's plenty strong enough to charge up both batteries quickly if they happen to both be low. And yes, the NOCO "remembers" its last setting, so when it gets plugged in it's already at 24v without me doing anything.
Third, I bought a "battery balancer" that maintains the charge level between the two batteries. This keeps the charger from charging one battery to "full" and leaving the other partially charged. It has the same effect on the voltage coming from the alternator, which is a plus. Batteries wired in series can have this "one isn't charging fully" problem.
I put all the electronics and connections in a waterproof box (a Harrdig-like case) with grommets that allow a 120v plug (to plug the system into the wall), and the heavy-duty cable with the NATO slave plug attached to exit the box. Also, I have another 120v plug on the end of a cable that reaches to my engine block heater.
In the summer I plug the truck in overnight about once a week, maybe every other week. In the winter I plug it in every other night or third night. Sometimes I plug it in to the block heater if I know I'm going out the next day.
In two years the batteries (cheap Wallyworld ones) have held up fine and the truck sat for three weeks this Fall while I was out of town and started right up, so I know this setup isn't killing the batteries.
Hope this helps.