Take your engine battery to NAPA have it charged and tested, in that order.
Replace you house battery disconnect with a DC-DC battery charger. Not cheap, but it is 100% automatic and you can be sure that it will charge while driving. Also you need to verify that your converter is output sufficient voltage to keep the house battery charged. You should have about 13.4V at the house battery when plugged in. If not, there are a lot things to check.
Water heaters are tricky. There are multiple safety features that will prevent it from starting. Each needs to be tested.