Battery losing charge

I don't think that year of TT would have a factory disconnect switch & most new ones don't either from my understanding. What you're looking for would be an aftermarket switch installed by the previous owner, if he did one at all. Normally they are located at or very near the batteries & either connected to one or the other battery leads. I don't think you have one, since it's near impossible to miss seeing it. Google "battery disconnect switch" & you'll find many different ones. I would recommend installing one, it's purpose is to eliminate any drain, parasitic or not when in storage I simply used an "Anderson" single pole connector on the negative side of my batteries.