Fresh Water Tank Problem in Itasca Navion

My wife & I recently purchased a 2008 Navion IQ. We've rented RV's out west a number of times for 10-12 days at a time so we have some RVing experience. One of the key reasons we liked this unit is we prefer to do a lot of dry camping and this unit was supposed to have a 40 gallon freshwater capacity (according to the specs that came with the unit.) I fill the tank until it overflows each time we head out on the road, but by the time we arrive at our nightly destination the tank level reads 1/3rd full. I have looked for leaks and found none so I thought the problems might just be defect sensors. But this weekend we ran out of water after the first day having used maybe 10-15 gallons. After inspecting the tank after filling again (using a flashlight to view the tank level)it appears the tank never fills all the way. In addition, we seem to lose water through the overflow valve when it sloshes around on the open road. (you can see signs of this when we stop for refueling, etc.) I've checked after filling the tank until it over flows and leaving the unit sit for 2-3 days. There are no signs of leaks. I haven't been able to measure the tank yet to verify the capacity but even if it is a 32 gallon tank, as in the DL, we still aren't using anything close to that amount of water in 1 day. The only other things I can think of is the tank is not installed level and is tilting forward so the over flow valve is lower than it's supposed to be (hard to understand how this could occur since the tank sits on the floor), or there is a design flaw that did not raise the overflow valve high enough to avoid these issues. If it's a design flaw, someone else must have seen this issue before. The only fix I can think of if this is a design flaw is to create a "gooseneck" that raises the height of the over flow valve so we can force the tank to fill all the way and make it much harder for water to slosh out while driving & braking. Has anyone else had this problem? Suggestions?