Heater not getting warm on a 2013 Ram 1500

air pockets ? top off the reservoir after a run ?

You have a air lock in the heater core, you need to burp the cooling system, it can take a while in some cases you have jack the vehicle up to get all the air out, I think the heater core in this vehicle sits higher than the engine. You may want to put a flushing tee in the heater core return hose to help get the core bled. I hope this helps