solar charging question - adding additional battery

OK thanks very much for the advice.

No problem at all. Could be a slow charge if the controller has already moved to float. Connect in the morning during the first hour of sun.

I have done it many times,....pull the boat alongside the MH, parallel the trolling motor batt to the house batteries, and fire up the genset, works great. Genset, solar, same difference.

Its a 55amp hour. Optimal blue top AGM. It would be hooked up to the trailer batteries with like jumper cables. It would be about 4 or 5 times a summer when im on long trips where i need to recharge the boat battery in order to fish more! It lasts for 2 or 3 day trips but then starts to die and needs recharging. I have a 60watt portable solar kit i could hook up to it, but using the 400watts from my trailer would be so much faster to recharge.

It will work. If you only need to do it once then go ahead. It's not an ideal way to charge the battery.
You'd be better off disconnecting the solar from the existing bank and hooking it just to the boat battery for a while.

You won't overcharge the one while charging the other--the voltage of both batteries has to be the same when they're connected together in parallel (well, excepting differences due to wiring voltage drops in the interconnects). You'd see some current flow (particularly at the onset) from the charged battery bank into the discharged one as the voltages equalize.

Ideally I suspect the best approach would be to connect the discharged battery only to the solar charger until it is charged to about the same level of charge (voltage) as the charged (and presumably now discharging) house bank, and then connect them together. However, unless the discharged battery is quite large in comparison to the charged one, I can't foresee any great trouble with connecting them together from the start. Do use sufficiently heavy interconnect wire for safety.