Originally Posted by DancinCampers
I would check inside your cabinets for a leaking backflush vacuum breaker valve.
A vacuum breaker valve shouldn't leak at all, and if it is faulty, it would still only leak if the line to the tank flush was pressurized. That line should only be plumbed to a female hose fitting, not hooked into the fresh water system that could be contaminated by black tank liquids.
Was the RV winterized? I'd suspect a leaking toilet valve. Any water left in the toilet could have frozen and cracked something near the toilet. Again, toilet paper or paper towels wrapped or placed under possible leaking points can help reveal the leak.