Did I ruin my water heater, please help!

The dealer was full of it. There are enough little power drains on a TT to completely drain a battery in just a couple of weeks. Your battery is now permanently impaired (the amount of charge it will hold has been reduced). It might continue to give you enough juice for lights, fan, water pump, etc. for a day or two of dry camping, but it might not. If you prefer to camp with electric hookup, it'll be okay for that. Leaving it plugged into shore power or a small solar panel while in storage is generally considered to be the best way to go, but some folks do disconnect (and sometimes even remove) the battery entirely and it will then keep its charge for a couple months (but temperature is a factor, and also dirt on the battery top can transfer a trickle discharge).