There's probably no reason not to fill your tanks when you hit the road. Your RV shouldn't be overloaded, but this is something you should check. With a full tank of gas and water and propane, and all your normal camping gear, stop a truck stop and use their scales. You will get front and rear axle load numbers. Did ours and we are not even close to rated weight, but we have a 29-year-old RV probably made before the advent of slide-outs.
My dad one time got stuck on the road when his RV broke down, and he had not put any water in his tanks. State Patrol dropped off some water for them.
We do not drink from our fresh water tanks. We carry bottled water for drinking and cooking. The fresh water tank is for the toilet and shower and teeth brushing.
So, you don't have to use your fresh water tank, but you paid for it and it's a nice amenity so why not?