You sure can drive the RV with a full fresh water holding tank, but you should only do that if you are planning on doing some Boondocking (camping in a location that has no water connections, power connections, or sewer services).
If you are planning on going to an area that has all the connections and services mentioned above there is no need for you to travel with a full fresh water holding tank especially if you are interested in getting the best fuel mileage you can in your RV.
One of the factors that affect fuel mileage in a motorhome or RV tow vehicle is the loaded weight of the motorhome or the trailer you are going to tow. 40 gallons of water adds an additional 334 pounds of weight to your RV. Now that may not sound like a lot of additional weight, but it will affect your fuel economy.
Any effort you can make to reduce the loaded weight of your RV will have an effect on fuel mileage. For example I urge all RVers to always dump their grey and black water holding tanks before hitting the road, regardless if they are full or not to reduce the loaded weight of the RV.
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