I had both problems, the gauge pegged full until I ran out, and it would only take 12 or so gallons. Turned out both sender and pickup were bad....on both tanks!!
So I did the sun-pro universal fuel sender, pick-up tube, and filler neck mods on both tanks. The front gauge reads pretty accurate, but the rear doesn't move untill I have less than 1/2 tank...thats when I switch it to the front where it is accurate and I have a little reserve in the rear.
I still cant use the big diesel pumps, but I can hold the auto pump on the lowest setting and still fill up 15 mins shorter.