OK Tommy,
One last idea or concept really, cuz, NEVER say Die! Currently the issue is that a siphon can't be achieved because the delta between the tanks is too close. Correct? So the solution to that issue would be A: raise aux. tank or B: lower main tank
Digggin holes sucks without a backhoe or a chinaman, so lets focus on A: raise aux tank. Probably wont take a lot but lower aux must be higher than upper main. Water is heavy but luckily yours comes with a motor. Soooooo...... how bout a couple ramps to drive up or back up, or a "stand" that raises the tank in the bed just enough or both. Also, If you can get the hose down the filler neck into the water and drain or use some from main tank that will pull a suction and remember "water seeks it's own level" so it may want to all go in lower tank if you don't stop it. You can also use some of the main water to backfeed the hose and push all the air out, then the water in the hose will be heavy and no air and right at the filler. Air will kill a siphon. OK, Pop a top on some genius fluid and see what you come up with.
McDave