Engine temperature gauge reading consistently low

I noticed my truck was running very cold. The heater works fine, but the engine
temp only ever gets up to about 1/4 hot at the highest. I replaced the engine
temperature sensor above the thermostat but that didn't fix the problem. I
noticed that when I changed my coolant and was trying to get air out of the
system, once I topped off the radiator no more fluid got sucked in from the
radiator even though it was running for 15-20 minutes. The temp gauge was
still reading cold. I'm wondering if maybe the thermostat is stuck open? I don't
know what else to try and don't want to do any damage to the truck. Please let
me know if you have any suggestions? Thanks!