Battery trouble - 7.4 volts to house batteries...

I'd disconnect your batteries and inspect them for damage, check the water levels and try charging them with a regular automotive charger.

Worst case scenario is one or more of the batteries is ruined, and has formed a short circuit. This is bad because it will heat up the battery something fierce, and could cause rupture, making a big mess.

It could also be something simple like a corroded terminal on the switch, or on a battery, not letting the full voltage through.