need some electrical help...

ok guys heres the deal. the squeal is coming from the propane sensor (only when i unplug the trailer from shore power.) but...when i first unplugged, this time it took a few minutes to start squealing. the red light is on on the propane sensor, indicating it time to replace. it will not mute when i press the mute button. the batteries initially looked ok, but as soon as it started squealing, they looked dead, with widely different readings - the inverter indicated 13.2 volts, but the solar panel indicated 4 volts. i hooked the battery charger up to the batteries and it indicated something in between, but it would not charge - it just shuts down and indicates an error. also, the fuse from the battery looked melted (the plastic) but it was tough to see if was broken - when i pulled it out that basically destroyed it. i tried to put another one in, but it was very difficult since the fuse holes seem a bit jammed up. discuss...