Sounds like a combination of things. First off, I would not be too quick to depend on the temp gauge for accuracy. I have always considered those more of a 'cold-normal-hot' gauge.
If your having a heater temperature issue, it is not usually a thermostat problem, but more likely either an air lock in the heater core, or a plugged heater core. If you check the heater hoses (after the trucks been running awhile) for warmth, this a good indication of what's going on. Under normal operation, both hoses should be 'uncomfortably' hot right where they enter/exit the heater core (at firewall). If one is quite warm and other slightly warm, then chances are its an air lock. If both are slightly warm....or cold, then likely the heater core is due for replacement.
You 'Might' be able to get away with a good cooling system flush, and I would recommend this anyway, but if the heater core is so bad that it is plugged, you might as well replace it now, because if the flush opens it up, it's likely going to start leaking in a short while anyway.
Good luck with getting your heater working for winter. No fun driving around in a cold vehicle.
Interested in one of my mods?
If it's underlined, it's a link to that mods thread.
2010 Ram 1500 Hemi Sport, 3.92, Leather, NAV, Alpine, Sunroof, Trailer tow, Brake Contoller, Rear Cam
Mods - 3M clear protect, Decals removed, Electric Tailgate Lock, Tow Hooks, Skid Plates, Bed Rug, Pace Edwards Bedlocker Electric Tonneau, Timbren Rear SES, Tinted Tail Lights, Debadged, Grill Blacked Out, Shorty Antenna, Black Chromed Emblems, Lightly tinted front windows, DS Predator, Cabin Air Filter, K&N Drop In, Mufflex (Magnaflo) DOD24M, Sonnax line pressure booster, Volant CAI