Hello and welcome to JustAnswer!
If the engine is getting up to full operating temperature the first thing you need to do is flush out the heater core. Remove the hoses at the opposite end, leaving them on the core. Run a garden hose into each hose for a few minutes to flush each direction. Do this a couple times and put it back together. Top off the coolant and bleed the system and see if you have heat. Your heater core may be partially blocked internally with buildup and this will break alot of it loose. This may very well be all that you need to do to get heat again. This would be a completely different type of heater core problem, you won't see or smell any leakage.
If that doesn't fix the problem then it would be time to roll the dash out and see what's going on with the blend door cable. This is the cable from the control head on the dash to the top of the heater box. It may have broken free of it's mounting on the heater box or slipped out of it's adjusting clip, or the blend door itself may be sticking in the heater box and not able to move to divert air through the heater core.
If you take the hose off at the thermostat that would be a radiator hose, we need to catch the heater hoses going to the heater core. Look at the firewall behind the air filter box and you will see the two heater hoses going through. Follow these back forward. One will run up the right side of the intake and will end up entering a nipple on the intake to the right of the thermostat. Often there will be a junction in this hose around the middle of the intake area where you can remove the hose and will be able to flush from there. The other heater hose runs along the outside edge of the left valve cover, and then attaches to the metal tube coming off the left side of the water pump. Remove this hose there at the tube. Removing these hoses there instead of at the heater core will be better for flushing because you'll be able to put the hose right to them, and it will be much safer than removing them at the firewall. The sensitive copper tubes of the heater core can be easily damaged when removing the hoses at the firewall.
You're welcome, and good luck with your truck!