What is the weight difference of water in a jug you purchased or the same amount of water in your fresh water tank? Time is up.....nothing!!!!
You are missing a great deal of functionality of your RV by not using the fresh water tank. Fill the tank (maybe not completely full if you are watching weights) then you can flush the toilet and take showers!
Staying at a truck stop? Why? There are so many better choices; even the Walmart parking lot will be better! Find nice city and state parks, choice boon docking locations, and even commercial campgrounds.