Keeping the fresh water tank full aids trailer handling as the COG [center-of-gravity] is lowered. Having water available is rather the reason for an RV. How to treat, maintain, etc, is part of other threads. And, there is no mpg penalty.
A "fulltimer" of any description ought -- in my opinion -- be familiar with using the TT without any utility hookups. See what is involved in three-days living and up to two weeks. There are many places one can camp . . sometimes whether one wishes to or not. Knowing what one can and cannot do is valuable.
1990 35' Silver Streak Sterling
; 9k GVWR.
2004 DODGE Cummins 305/555; 6-manual; 9k GVWR.
Hensley Arrow. 10-cpm solo, 18-cpm towing
Sold: Silver Streak Model 3411