We have had trailers, a motorhome, etc. Some have had plastic type fresh water tanks, one galvanized ~ had one plastic type in a sailboat. In the trailers we always empty the freshwater tanks at the end of a trip (or shortly thereafter) and sort of let them air out. Otherwise you get icky tasting water! On our boat it was much more difficult to find fresh water when out at sea.... the marine stores sell a product called "Sweet Water" and it helps to keep the water tasting fresh. But we aren't talking about weeks and weeks here...
Besides, the inevitable leak will appear sometime in the water tank. If it is filled with water it can slowly wick into whatever is nearby without your knowledge until you step on the floor and it is squishy.
If you fill and empty you might spot a leak quicker. (Maybe only wishful thinking!)
Good luck! Have fun, and go camping!
Mrs. NorCal Bambi (traveling in S Tardis)