heat issues

Hello. The subject truck has ~50k miles. Air flow blows when the heat/AC system is turned on, just no heat at all. Initially, the reservoir for the coolant was low, topped that off and heat returned. A couple weeks later, no heat. Where would you all suggest the troubleshooting starts...

I started noticing issues on the 2nd. I started the remote start to heat up the truck at night and it was warm but not as hot as it should be for sitting 8-10 minutes. There was frost on the inside of the window that would not dissipate despite the heat being on the entire 15 minute ride home...