Yep, those overflow hoses really suck. Many of us have put valves on them but you have to remember to open them up when filling or you will blow up your tank.
My Greyhawk has 3 hoses. I think 2 of them are overflows which tell you when the tank is full and the other may just be a vent. I found that I would lose almost 1/3 of the tank during transit before I put the valves on. Now I arrive with a full tank which is needed when dry camping.
I also recently found out that the water will flow out before the tank is full if you are filling at full pressure. On my tank I could look through the wheel well and see the amount of water in the tank. I believe that filling with full pressure forces the water through the hose sooner than when it would start its normal siphon action.
Now my sequence is open valves, fill fast most of the way, slow filling, shut valves when they puke. Then when I set up at camp I will open the one vent valve. This has been working very well for me.
I mounted my valves on a little piece of angle aluminum on the bottom edge of the rig.
Retired electronic technician (45 years in the field)
2017 Greyhawk 29W
solar & many other mods
wife (maybe I should have given her top billing)