Full or empty water tank

It will make a small difference in fuel mileage.
But, it's not enough to make me travel with an empty water tank.
Try it both ways and see for yourself.

I think it depends upon how large the tanks is and how "full" (Full) really is.

A 10 gallon water tank will make virtually no difference. A 100 gallon tank could make a difference.

I tow a travel trailer, not drive a motor home. But I do know when my fresh water tank is full (and it's 45 gallons), I can feel that driving the truck. When it's empty the trailer handles different. There's a certain amount of movement that occurs with the trailer. I can't explain it, but every time I've pull out of port and hit the high seas, I can tell within the first mile if I forgot to open the drain on the fresh water tank. I'll stop first chance I get and let it drain on the road. Within a few miles, the to experience is different as the tank empties out.

About fuel mileage, I really don't know. As far as weight, if you have a 50 gallon tank (and 1 gallon of water weighs approximately 8 pounds), that's 400 pounds of water.

If you carried two passengers with you that weighed 200 pounds each... how much would that affect your fuel mileage? Probably not enough to mess with. So is the case with a full tank of water.

The difference (in my opinion) is how the trailer handles when towing. With a motor home, it depends upon how solid the suspension is, how heavy the motor home is, and how resilient it is to movement inside the coach. If the MH is a behemoth, even 100 gallons of water may be nothing compared to the over all weight of the coach.

Like he said. We don't worry about the mileage and keep the tank full when traveling. Don't have to worry about finding a place to fill up or a campground whose pumps have stopped working (only happened once) AND the tank can be used when stopping at Rest Areas.

Mine RV actually rides a better about 1/2 full. Another thing is...we had reservations at a place and when we got there they had the water shut off. Someone pulled out with their hose still attached. Broke the water pipe and a 4x4 stuck in ground next to it. They were in process of fixing, but we were glad we had our own water. Must of been weak from getting pulled on before, I wouldn't think a hose would be strong enough, but it was. Also good in case you want to flush toilet on the road.

Virtually no difference at all, UNLESS, you are close to you're to your max weight. It's the front end wind loading on your rig that has the most effect on mileage... think about your engine having to push/pull a brick through the air.

Once the weight of your rig is moving, the engine isn't going to notice the extra few hundred pounds of water. Speed has the most detrimental impact on mileage.
I look at a full tank of fresh water the same way I look at seat belts, you might never need it, but if you do, it's great to have it.

Several times we've arrived at campgrounds where they've had a power failure, and the water system boost pumps were out. It was great to have our own water then.

Wind resistance has MORE affect on mileage then weight.

Got caught ONCE w/o water being available (CG water line broke) and my fresh water
tank was empty.
ONCE!

I travel with full tank at all times. Full tank has no measurable effect on fuel mileage.

Cummins has an analysis of weight vs mileage. For the number of pounds you are adding even with a 100 gallon tank you may not be able to see any difference.

Far more important is having a head wind--or a tail wind (may you always have tail winds!).

No issues traveling full. My DP seems to ride better with a 1/2 tank or more.

I took my new trailer on the road, and it felt "funny" like a back-and-forth motion; I didn't like how it was handling. When I got to the campground, my daughter pushed the fresh water tank sensor and it was full! Apparently the dealership had filled it to check that everything worked, and didn't empty the tank. I emptied it, and on the way home towing was a breeze. I don't want to haul an extra 400 pounds of sloshing water.