Need some guidance on fuel tanks - 85 Eldorado President

My best friend just picked up a sweet 85 Eldorado President. 34 ft Class A. Now we can camp together again. Everything works great. We fixed a couple leaks and updated a few small issues but no major work needed. However, last Saturday he ran out of gas. It has two tanks. The first one got low and he flipped the switch he believes to be the one to go to auxiliary tank which on the gauge says is half full and boom, dead on the side of the road. If he flips the switch on the dash it shows the fuel in the tank according to the gauge but does not seem to switch to pull from that tank.

Any ideas on how this should work? I haven't really had a chance to dig into it yet but I don't know much about dual tanks as mine is a 38ft TT. I am sure I could tear into the dash and start tracing wires but I thought I would post the question here. We are trying to get ready for Sturgis next month and being able to use both tanks would be awesome for range.