Since the gauge is about right, that lets out the t-stat being open. Air in the system? Yep, changing the heater core is pretty intense. Most of the dash has to come out to get to it. Good luck.

 

sounds like the Thermostat ...if the temp gauge stays low, id replace the thermostat, if the guage is not low,
id bring it to the dealer!!!

 

Not the thermo if the gauge is in the middle, middle on the Ram is normal operating temp, actually just ever so slightly to the left of middle.

I would guess either cap or there is air in the system that needs purged.

The Ram also takes a while to warm up when really cold (below 20).