Fresh Water Tank Leak

Never seen a Fleetwood coach that the water tanks cannot be removed. Difficult, oh yes, but not impossible. You have to remove the componants between the tanks and blocks. Then you should be able to slide one or the other back or forward to slide it out the side opening. Once one is out, the other can be removed through the same side opening. :)

You have an overflow hose on your coach, as do most RV's. Find that overflow hose, which on my Monaco is on the opposite side of the coach. Once you have the overflow hose, plug it off. Then if you have a manual fill valve, which on mine is above the shore water hose connection and allows me to dump 5 gallons of water in from a container. Make sure that hole is plugged or the cap is on tight.
Now you should only have the fill valve, that is used to connect to shore water. With the tank empty, as you say it is, use a fill hose to pressurize your tank with a low pressure and see if you can hear a leak somewhere, or if it will hold pressure of no more than 20 psi. If you hear air escaping try to find the location. If it holds air I would suspect overflow hose siphoning.
The overflow hoses have been known to siphon water out of the fresh water tank on some coaches, which it did on mine. I just left the overflow hose plugged and use my manual fill valve as the vent. No problems since then for the past five years.