OK the schrader valve is to fill the front shocks with air on the Venture Standard (No CLASS). Only use a hand pump, do NOT use a compressor, you will blow the seals out!!!
OK very common problem with the gas gauge, your sender unit, located on the top of the tank and bolts on the top, is most likely screwed up. The rheostat is probably warped. Remove the sender unit, and behind a metal shield is the rheostat, it's slider is connected to the float. The rheostat winding is supposed to be flat but chances are yours is now bowed, You may or may not be able to fix it. Sender units still show up on Ebay but it is a crap shoot to find a good one. Study the unit, the tension of the slider is sort of adjustable via a phillips screw adjustment...