The first time I went dry camping with my KZ, there was not enough water for a second shower. Turns out that most of the water spilled out in the 80 miles to the campground. The tank is vented from the top edge with hoses towards the ground. When rounding a right hand curve the spill would continue till the road straightened out. Running the vents up and out the side, higher than the tank, cured that problem. You should know that tens of thousands of trailers were built that way and it was not only KZ. I followed one like that into a campground just last week and mentioned the problem, he was having none of that about his great RV. I guess those following, especially motorcyclists, are of no concern.
In addition, the water pickup hose was attached to the side of the tank and it would suck air as soon as the water level dropped to the top of the hose fitting. This alone left about 25 percent of the water unusable. I cured this problem by teeing into the drain hose at the bottom.
Now add in the fact that the sagging tank was supported by only three straps, there was even more capacity wasted. I added more straps to make the bottom near flat.
Had I not done the above things, my capacity was between 0 and 10 gallons, I now have use of nearly 100 percent of the 40 gallon tank. It is surprising how far 40 gallons will go when you actually have 40 gallons and not just a number in a brochure.
I reported the safety aspect to the NHTSA, but they don't care. A vehicle that is losing it's load and water cooling the brakes on one side is of no importance. Vehicles following suddenly hitting a wet road and getting a shower is fine, I guess.