Fresh water tank getting fresh water?

If you are in the city fill mode the pump may run and pump water from the fresh water tank. If the pump is on and the shore water pressure is lower than the water pump cut-in pressure it will run and take water from the tank.
At one RV park that we spent all or part of the winter in for several years, the water pressure varied from 20 psi or so up to about 35 psi depending on whether the well pump had just pumped the well pressure tank up and the number of users. When we took a shower we always turned the pump on so we had a decent pressure when the shore pressure dropped.

The idea of traveling with an empty tank as not as foolproof as many think.
The weight will have negligible effect on MPG's.
Location of the tank can effect balance and tongue weight of the trailer. A partially full tank with water sloshing around in it can contribute to the tank becoming loose from mounts and falling out onto the ground.
I always travel with a full fresh tank

Thank you all for the quick response - I most appreciate it!

GOOGLE stale water

As stated above, correct! When hooked up to city water you are not pulling anything from the on-board fresh water tank.

We travel similar to you, with very little water in our tanks. When doing a stop for overnight (we always stay in a park, never boondock), I pump my water from the fresh water tank. For quick overnight, I'll fill the water tank at the campground when we first get there. I quit using city water hook-up a long time ago, because of leaking hoses and unknown water pressure. The water pump on-board will not exceed the pressure capacity of the plumbing, but you never know what to expect when attached to city. And I simply do not trust pressure valves and don't want to spend a bunch on one either.

So in the morning, whatever remains in the water tank, I go ahead and drain it, leaving just a few gallons for traveling if we need to flush the potty. We do sometimes drain it completely and carry a 6 gallon jug with water we use for flushing.

When parked at our destination, I fill the fresh water tank also, and only drain it down when leaving.

This keeps the water in the tank fresh. I've never had anything growing in my tank, hope I never do, and even if you drain the tank, there's still moisture in there. But when traveling, I want my tanks as empty as possible. I've heard too many stories about tank failure (falling off) on the road. Nope! I'm not taking that chance.

Water doesn't go stale - but the chlorine will eventually lose it's effectiveness but that takes a long time. Many RVers will sanitize their FW system with bleach once a year and don't worry too much about how old their water is.

Correct.......

'Fill' position fills fresh water tank
'Pump/City Pressure' supplies RV plumbing System with water

Using on-board pump is only thing that sucks water out of fresh water tank


Couple suggestions........

Fill Fresh water tank and use pump
We use to travel with only 10 gallons or so in fresh water tank and just use city water connection when we got to a CG.
That was until we got to CG and their water system went down (main line break)
We didn't have water.....add to go buy 5 gallon bottles

From then on our fresh water tank was always FULL (the additional weight didn't do anything to mpg)

And when using pump it will let you know how well your RV plumbing system is functioning...........any drop in pressure and you will hear pump 'Burrrrrr'

Then you know you have a leak.......could be minor or worst.
BUT on city water you don;t know until it becomes WORST

Shore water does bypass your fresh water tank and 2 weeks should be ok. If your hooked up why not drain the tank and get fresh water when you notice anything weird in the fresh water tank. Most places don't charge for water usage.

We have sometimes noticed a strange smell and that is a warning to drain the water and sterilise the tank and start fresh.