Auto-Fill - Pool Pond Spa Water Leveler PVC Auto Fill 3/8" Float Valve

Any thoughts on this? I'm thinking I could plumb this into my fresh water tank with a qd and I'd have a home-made auto fill. The biggest problem I see is the nut loosening itself and filling the whole vehicle with water. Aside from that scenario I think it might work.

View attachment 27130 http://www.amazon.com/Pool-Water-Leveler-Float-Valve/dp/B005SHHT1I