I have traveled with my fresh water tank full and with it empty. I have also been reading up on the practice. Some say to fill them up and take off. Others warn of dangers with driving around with full tanks.
The ones that say fill them up also say that they have never had a problem with this practice. Although, some admit that their gas mileage is decreased due to the added weight. In my trailer the fresh water tank capacity is 42 gallons. That adds about 350 pounds to my trailer.
The campers that talk about running with empty (or nearly empty tanks) relate stories of not only poor mileage, but poor towing performance of the trailer when they were carrying full tanks. They point out that this is more noticeable when the fresh water tank is located behind the wheels. I believe that my black tank is located above the axles, the gray is in front of the wheels, and the fresh tank is behind the wheels.
The issue is that the added weight causes the trailer to be light on the tongue and ‘drag’ on the back. This can result in more sway with this heavy weight sloshing back and forth. But, there is another hidden danger that people have documented.
The tanks can fall off because they are too heavy. There are stories out there of people losing their tanks because weight is too much for the straps that hold them in place. The fresh or even the gray falling off is bad enough, but imagine the hazmat situation caused by the black tank falling out on the road.
So, unlike what I did in our trip to Interstate Park in 2021, I will be traveling with empty (or mostly empty) tanks. Dump stations and other locations will allow a fill of the fresh water tank when we arrive or a short distance before the campground. Yes, there will be times when we have to travel with full tanks, but hopefully that will be for a small number of miles.
As an Amazon Associate I earn from qualifying purchases.
Man of Wy 
How about 40-60% full or full because many areas hard to fill up when you need water and at my age I like my bathroom available and use my galley while travelling. If you travel a short distance ok empty. I also drive a class C Motorhome so can use easily use amenities along the way.
Like I said, there are many ways to do it. I do leave a few gallons for that stop along the road. Also, I am towing a trailer, you have a Class C. They are totally different in construction and handling on the road. So, it is left as a matter of personal preference but I have at least informed people to look at the issue.
Well I guess but I thought you were worried about fuel efficiency.I also flat tow and get asked all the time about how many miles to the gallon or litres/kilometres but if you drive to tow your least problem is fuel economy because you are already eating your gas up. I also found with flat towing or carrying water in tanks I’m more stable with winds and passing trucks.