Drill a 3/8 inch hole in the floor to let any water out that might over flow. Then pull overflow tube up above the tank. If it overflows water goes out the 3/8 hole. But it won't siphon out !! The hole left in the floor where you pull your overflow hose up will also drain any water that come out the tube. Just sand off all the excess silicon sealant that's there.
Then cut a 8" or 10" hole in the wall between the water compartment & the storage compartment. Then you can open the door in storage look right at the water tank and watch it fill. Too hard to unsnap the water compartment door. I'll attach a pic of the hole I cut.
In the pic you can tell that my water tank is empty. And I also have a lid that you can screw into the hole to close it up if you would want to.