There is much debate on whether to travel with your RV freshwater tanks full, half-full, or empty. Everyone seems to have their own opinion, and most of it is based on perceived logic or personal experience. There seems to be no hard and fast rules.
One common piece advise to RVers to travel with your freshwater (and gray and black) tanks empty, so as to not adversely affect your mpg. But, how much does weight actually affect your gas mileage? The rule of thumb is that every 100 pounds reduces mpg by 1%. So, if you have a large 100 gallon freshwater tank, it would weigh 800 pounds, and theoretically reduce your mpg by 8%. If you normally got 10 mpg with the freshwater tank empty, you would get 9.2 mpg with the tank full.
My RV has a 30 gallon freshwater tank, so it could add 240 pounds to my RV, and hypothetically reduce my gas mileage by 2.4%. If I normally get 10 mpg with the tank empty, I would get 9.76 mpg with it full.
We can see that for short trips, the effects would be negligible, but for long trips, the effects could add up. If you had a large rig and got 5 mpg, and drove 1000 miles with a full 100 gallon tank, you would hypothetically burn an additional 16 gallons of gas. My smaller rig, with a smaller tank, should burn an extra 3 gallons of gas on that same trip.
But the problem is, other variables can nullify this theoretical rule of thumb. The size of vehicle, or the size of the motor, can greatly affect this paper guideline. Smaller cars, and smaller engines, are more affected by this rule.
If we follow the hypothetical rule of thumb, we would cut our mpg in half by adding 5,000 pounds, and should theoretically double it by removing 10,000 pounds. Or, adding 10,000 pounds to a vehicle should cut its mpg by 100%, giving us 0 mpg.
A loaded semi-truck and trailer gets about 4 mpg on average. If we remove the average 35,000 pound payload, it should get about 14 mpg. If we removed a maximum payload of 80,000 pounds, it should get 32 mpg. But, in truth, the average empty semi-truck and trailer gets 6 mpg. So, looking at it the other way, if an empty semi gets 6 mpg, adding 35,000 pounds would reduce the gas mileage to a minus 15 mpg. While this hypothetical rule of thumb may work somewhat for small passenger vehicles, we can clearly see it is useless when applied to much larger vehicles. So, the truth is, that extra weight probably won’t affect your gas mileage noticeably.
If your RV is already overloaded, you should definitely avoid adding the extra weight if possible. Extra weight can cause extra wear and tear on your RV. Extra weight can put extra stress on tires and on the power train. It can also affect your driving ability. A large percentage of RV accidents are caused by overloading the RV. The convenience of having water on board, is not worth the risk of a wreck.
If you regularly travel with full tanks, you may get into the bad habit of always keeping water in the tank. Any water sitting in a tank has the chance of building up bacteria. Freshwater tanks should be kept fresh. They should be routinely sanitized and emptied. See- HOW TO FILL AND/OR SANITIZE YOUR RV FRESHWATER TANK
BENEFITS OF FULL TANKS
So, what benefits, if any, are there for driving with full freshwater tanks?
The problem with traveling with an empty freshwater tank, is expecting to get water when you arrive at your destination. We should be old enough to know that things don’t always go as planned.
What if you have trouble on the way? You could be stranded with all the conveniences of home, except for water. What if water wasn’t available when you got there? We recently stayed at a campground which had just gotten the water turned back on after two weeks of repairs. They said to let the water run awhile before hooking up, and only drink bottled water. Our tank was full, so we didn’t even hook up. No problem.
What if you just need to go to the bathroom before you get there? Or maybe want to wash your hands? That’s why many RVers travel with ¼ or ½ tank of freshwater. They feel there’s no need to haul a full tank of water, but want some for convenience or just in case. We should also consider how much a tank holds. If you have a 100 gallon tank filled to only ¼, you have almost as much water as my full 30 gallon tank.
As stated before, there are no hard and fast rules that determine the best route to take. There seems to be no definite right or wrong. It appears to me that this all boils down to personal preference. There’s enough range between full and empty to at least lean toward your own opinion.