Have you always used this system for fueling? That is, you periodically refill you main tank, and then add the "new fuel" to the auxilliary tank in the back of your pick-up? If so, I'd continue to do that so that your fuel is relatively fresh.

IF you plan to keep it "cooking" there in the back of the truck, it's going to get pretty hot -- then cool off every night in the cold desert nights. That's not a good idea, even if it's relatively dry.

IF you decanted the fuel through a filter that would separate out the water and any particulate, then it would probably be okay.

If you take the fuel from the gravity hose feed and put some of it in a glass container, then I think that you would be able to see if its clean and pure. if that's the case, then I think you should be okay -- until it's not. You can dump the CLEAN PURE fuel back into the tank.