On my first trip out, I ran into the same problem. I filled my tanks before leaving home, but by the time I got set up in camp, I was down to about 2/3 full. So, before leaving home for my "full-time adventure", I picked up two 3/8" hose barb x 1/2" MPT (male pipe thread) brass fittings, two screw-tight hose clamps, and two 1/2" FPT x 3/8" compression straight shut-off valves. I cut about 2" off each hose (they already hung down about an inch below the axel), slid the hose clamps over the ends of the hoses, slid the hose barbs into the ends of the hoses, and tightened down the clamps. (Hint: a couple drops of dish soap makes the barbs go in easier.) Then threaded the shut-off valves onto the barbed x MPT fittings.
I, also, leave the valves open until water starts running out; then close them and fill up the tanks the rest of the way. Water does come out of the fill port overflow on mine. Then I climb under the trailer and open the valves for a couple of seconds to "burp" the lines. I arrived at both campsites will full water tanks, so this fix seemed to work well for me.