Your going to need to watch your truck carefully the next couple of weeks.
You don't indicate the condition of the coolant to start with, whether it was low, overflowing, or normal. I'm suspecting you may have had a coolant level issue, and it may be related to a blown head gasket, particularly since you have a cyl 7 misfire.
If they replaced the thermostat, they should have refilled it, and ran it for a while for the thermostat to open and purge the air, and topped off the radiator. They should have checked that your heater was putting out hot air, particularly since that was the problem that you took it in for.
I'd monitor the radiator and expansion tank level very closely, and make sure that you aren't loosing any coolant (sucked into the engine cylinder), or that your coolant is overflowing (gas mixture blowing into the radiator and exhaled into the overflow tank). If you are getting either of those symptoms, take it somewhere for a compression check on each of the cylinders.
It's possible that they just didn't run the vehicle and purge the air, but that wouldn't say much for their shop. If this is the case, then after a couple heating/cooling cycles your radiator should fill itself properly from your expansion tank, and everything should stabilize correctly.
Hoping for the best.......