Partially filling fresh water

Does anyone have a way to accurately measure how much water you're putting in (when you don't intend to fill completely)?   When I head out on an adventure I try to put in just enough water (say 10/12 gals) to allow me to stay overnight at a Walmart but save the towing weight of a full tank.  I have tried pouring water in a gal at a time - messy and very time consuming.  Lately I have timed how long it takes me to fill a 5 gal container and run the water that long (or twice for 10 gal etc.) using my hose and water tank filler attachment (a great invention BTW).  Works OK but takes a little extra time and wastes some water.

You don't need to be perfectly accurate for this by any means.  I've used the time method.  As you say, time how long it takes to fill a 5 gallon bucket and do the math.  Do that once and remember the time for next time.

There's some debate over how much extra water weight really impacts gas mileage anyway.  It has to have some impact but probably not nearly enough to spend a lot of time worrying about whether you have 8.4 gallons in the tank or 13.7.

TT

Another solution is to use your homes water meter and a helper with a cellphone at the meter and you filling with another cellphone.

I carry two water containers in the back of my tow vehicle, and tow with my r-pod tanks empty. Look up free surface effect to consider how free surface may impact towing.

I'm not an engineer, but my sense is that the free surface effect on an RPod is probably more theoretical than anything to be too concerned with.  There are times you might be able to feel the liquids sloshing as you go down the road under extreme conditions, but the likelihood of tipping the trailer is probably pretty durn small.  Maybe if you lost your brakes and were speeding out of control down a mountain road with a lot of curves, and you were probably going to die anyway, the free surface effect would add the last 3% of force needed to cause you to roll over.  Considering how low the tanks are, their shape and configuration, and volume of water they can hold, they may even act as more of a stabilizing factor than otherwise.

TT