You probably don't have much more than a 30 gallon fresh water tank. That's all I have in my trailer. Adds a whopping #240. Heck I weigh #220, so maybe I should stay home to save on this "MPG" savings you get?? ha, ha.. ;)
I always fill it up so I know I will have good water at my destination.. Even when I know the site has water hookups.. I don't hook up to the camp water either.. I just use mine and if I did use it up, I would have the camp water as a backup..
There has been a couple of times we got to the park and they had a sign saying the water was not working in the site for whatever reason, or like the pic above, where the water was so bad looking AND tasting, I would not want that gunk going thru my system..
As far as the locks on the propane and the battery... I have a long cable lock on the propane and I remove the battery when not in use. I have had a battery grow legs and walk away before.. And parked next to my house!
Good luck and you'll figure this stuff out just like we all did.. :)
Mitch