engine temp

[C-10volvo]

Well, as most of you know I have a 1984 c-10 305/700. After about a good 20 min. of driving, the engine temp is only reading a bit above 110f. Could I have a bad temp gauge? Or is my thermostat stuck open not allowing my engine to properly heat up. I put a new water pump on 4 months ago (bearings sounded shot) but everything looked normal. Any tips on how to check things out. Also while in making this.. I have a fuel gauge that has a mind of its own. Should I drop the tank and get a new sensor, or is this fixable in another way?
My truck has 52,000 miles on it, and has been passed down from my grandpa. I know it’s never been “mickey moused” and want to keep it running as long as possible. Ohh well, if I blow it up, in goes that 572. see how long that 700 tranny will last with 720 hp.