Both Batteries went down on me??

To clarify.. the refrig was running on propane. Only used the battery for the periodic ignition needed. The chassis is a Ford V10 1999. I only had the coach battery hooked up to the trickle charger in the basement over the winter. The chassis battery was left in the vehicle and appeared to be working fine up to yesterday. The two batteries have not been hooked up together. The coach battery is hooked up under the step going into the class C. I just checked it and found it and found that it was really low on water which is strange as I had topped it off in the early part of May. Do not know why the
water disappeared??

I have added a lot more distilled water and am charging the coach battery overnight to see how it is tomorrow.