Mr. Monk and the Weak Interstate Battery –

There is one fundamental procedure any decent garage can do(dealer?) to check out the battery which in your case had a weak cell.LOAD TEST THE BATTERY. A battery might/might start your vehicle or run your inside coach lights with your MARINE battery but that doesnt mean they have the beans to deliver the amperage hours promised by the warrantee. If the loading on the battery shows adequate spec performance under load, then the battery is ok. Yours would fail for sure. Why do I know this? When I bought my 2005 Winnebago Aspect, for the first few days I kept having trouble with both the Interstate truck battery going flat after a few days AND the coach Marine deep charge battery. I took it to the RV dealer, told them to load test them both and sure enough--both were below 12 volts under load. They replaced both free of charge. It is often possible also to buy a new battery which can develop a weak--below spec--cell,usually an outer cell. A new battery is the answer.I use a small solar panel plugged into my 12v outlet on the dash for the truck battery and now about to attach one for my coach battery to trickle charge both when I am not taking my PW Plateau for trips regularly.
AL