Hello again,
After doing more digging, we found a company that sells an upgrade to the original Ultra-8 monitoring system. Same cutout, and they'll modify the unit to use the same wiring harness as the original if necessary. They also sell new sensor kits and sensor repair kits, in addition to more traditional tank level monitors for metal tanks.
tankedge.com/products.html
So if anyone else with an Ultra-8 runs into the same problems we have, this is a good place to start!
Best-
Karen