Again, did you check the battery water level?
Did you check the breakaway switch?
And you describe a problem with the tongue jack. Does the problem show up with any other electrical device? When the tongue jack won't work, do any other electrical devices work or not work? Which ones?
If I could suggest ...
You are talking about several hundred dollars worth of batteries here. Don't skimp on a cheapie hydrometer. Garages use a temperature-corrected hydrometer with a glass float. It may or may not be more accurate, but it is only a few bucks more. I haven't looked in detail, but Amazon has one from Deka for about $18. As Dave noted, any auto supply store will have something similar. They just make me more comfortable.
I agree with one of the posts above suggesting you really need to confirm that there is an unintended current drain, and if so, where is the discharge current going, and how big is it? This can be an involved task unless you have the right tool. As Dave suggested, the tool to make it as easy as possible is a clamp-on digital meter that will measure DC current (all of them will measure AC, but only a few will measure DC). We had a discussion sometime back here on the Forum.
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Bill