Parked it and now, no start

Look in the darn tank and see. But yes, fuel will siphon itself to the low side tank

 

Fuel from the high side equalized into the low side while sitting there. So basically you're sucking air. Drop 20 gallons in the high side, get it primed and it should start.

 

If you have a shut off valve, located under the cab, look under cab near drivers door. It’s probably set to “both”. Set it to “left”, prime the filter if possible. or just try cranking it. It should start. Might need to prime filter a couple times, but it should start. A little ether if needed. Just to get it started, and fuel pumping good. If not, it’s completely lost prime, you can shoot air into the tank, using an air blow gun, and a rag to seal the opening. That will pressurize the tank, and push fuel into the system.

 

100% full? Other one shouldn’t be empty. Only happens when low on fuel. You probably have a problem with the fuel return line, or the valve itself. Ironically I have the same problem right now. I’m running on the left tank only. Both tanks draw, when set one at a time, but when I set them to “both”, my left tank doesn’t return, or the right tank doesn’t draw. Not sure. Right tank overflows. Have yet to figure it out.

 

What is the fuel pickup/return pipes setup on yours truck? If it's from the top of fuel tank pipe look for cracks on pickup tube. Usually crack is right under the flange due to vibration. It's sucking less fuel due to air intrusion but return is equal.

 

The filter housing shouldn’t be empty, because you’re parked on a slope. Something else is going on, with it. If yours has the fill cap on the top, just fill the housing full of fuel, and get it cranked up. Once running decent enough, pull it back up on level ground. I think, you have an air leak, with that housing. The fuel inside shouldn’t drain off. If so, you’d always be trying to start the engine on an unprimed fuel system. That’s just not how it works.

You’ve got to have fuel in the left tank. I can’t fathom you’re parked on such a steep slope the left tank is completely empty. And, fuel transferring between tanks wouldn’t have an effect on your filter. Each tank is vented, so no vacuum or buildup of pressure.