Ditto what AK said. Get a good quality guest charger and hard wire it into your system. You will need to hook up shorepower to do this and make sure you have good grounding. Most units are setup to automatically start charging when you insert the power plug. They usually apply a high charge initially and then drop to a float voltage of 13.64 or so. That way you don't boil out your water if you have wet cell batteries.
I have an inverter/charger system that I installed in my boat 12 years ago and haven't touched it since. Works great and keeps my big deep cycles charged.