When I replaced the t-stat in my 2000 Winnie, the original had a small
bleeder hole located in the top with a brass rivet to act as a check valve
I assume. The replacement T-stat didn't have this bleeder hole so I drilled
one with the smallest drill bit I had. This would prevent any air from being
trapped at the t-stat. You mentioned that when wire was pulled from sender
that gauge would drop. Try grounding the sending wire, gauge should peg the other way, if it doesn't stay pegged the guage may be at fault. If you can obtain an IR thermometer you can also get
some idea of radiator efficiency by comparing temp readings on the inlet at outlet.