Talked to Dakota Digital today... sender is supposed to have sender connected to one pole and ground connected to the other pole, NOT 12V. This depends on which sender you have though... I have the 30 psi sender, which gets the sender wire on one pole and ground on the other. I think the other two senders (with higher ranges, I forget what the are; 60 and 100 maybe?) have a different setup.
I was told to check the sender with multimeter. It should read 10 ohms across the two poles. I will check it when I get home tonight and see where we are. If that checks out, I will check the resistance across the sender and ground wires in the cab with the sender connected to see if it is consistent with the reading across the sender. I suspect that I may have a nicked sender wire that is grounding itself, although I was very careful when routing it. I will post results later.
Thanks for your help!