dont buy any parts until you prove the existing part is bad, of which there are many ways for the parts youve mentioned.
you can treat the gauge issue as a completely seperate issue to your mpg issue. two things come to mind:
-wiring from sender to gauge intermittent somewhere. you can prove this with someone sitting in the car watching the gauge while you either wiggle the wiring, or connect the harness in a way where it forces the signal to a false maximum (full high or full low). do this with the engine warmed up and the gauge should bounce around while you wiggle it if the harness is bad.
-gauge cluster solder connections cracked. this is not so bad to fix, but you have to pull the cluster to check it. this may be an issue ALONG with the harness being bad
you can check the sender with an ohmmeter, but I dont remember the proper resistance to temperature relationship, although its on the dodge garage and possibly the fsm
and even haynes.
you can unplug the sender and do something to the harness pins to force the gauge to read either very high or low, i dont remember how, but you can figure it out using the wiring diagram.
check, reset, and check your ecu codes with the check engine light.
as a crude test, you could let the engine "warm up" for 10 minutes, then unplug the tstat housing coolant sensor..the fan should come on, indicating the ECU sees a change in temperature
buy a OBD 2000 tool if you plan on keeping this car, $75 and its priceless, will tell you real time sensor data the ECU sees