Originally Posted by rgrugg
Go ahead and put water in your tank before you leave home... Fuel mileage is mostly affected by wind resistance, not a relatively small extra amount of weight
In some of the smallest trailers with tanks at the back, it can adversely affect towing stability, depending on what else you are carrying, and where. Water or no water, it's a good idea to check your axle
and tongue weights, loaded to go, before your first trip.
And while you're right that wind resistance is the main factor at highway speeds on flat terrain, weight does matter when climbing or descending grades, starting or stopping, passing or merging, so it always makes sense to pare unnecessary weight. Carry what is necessary, and if that includes water, and your trailer is properly balanced, fine. I live in an area of mountains and canyons, sharp curves, and gusty winds, so 80-100 pounds of water weight at the back of a 13' Scamp
is something I choose to do without.