Adam,
I think I'd be asking, or looking to see what else there are for loads potentially on the inverter, on the "ranch". Even when "unoccupied" there can frequently be phantom loads that add up (TV's, DVD/VCR's, overhead door operators anything with an RF remote, also things with clocks, rechargeable tools, flashlights etc. How about water pumping, heat tapes and the like ? Could be something else wrong, but I'm leaning towards Simon's thoughts, that the load is not meeting production and little things are adding up. If it were me, I'd suggest he dump the 1,500 heater.
It may take a series of deep charge/discharge cycles to recover them. Hopefully you still can.
Good luck, ~ks