Oddest thing happened today. I'm leaving to go out of town so I checked all the fluids for my wife and topped off the gas. All good. Two hours later, the heater was blowing cold air and there was no coolant in the reservoir. I couldn't find any leaks anywhere. I filled the reservoir and let it run. It didn't drain, but still no heat. There no coolant in the oil, but did see a drop under the dash. The air coming out of the vents smell like coolant as well. Ugh!

 

I hope you have stopped driving the truck, you have a major coolant leak and if driven you can, if you haven't yet done it, blown the head gaskets.
The truck needs to have a coolant pressure test done to find the leak.
If you want some more help with this, send me your number and I'll call you back.

 

Here's cooling layout from the RAVE tech manual. Note that heater core has no on/off valve, water is always circulating. O-rings can leak coolant on carpet. Heater core can leak, and takes some work to change out.

But if you lost enough coolant so level is below the heater core, you are talking like a gallon. If no external leak, let's check dip stick. A lot higher than normal "full" level? Look in oil filler on valve cover - got milk? (coolant and oil make milky look) May have to drain oil to verify no coolant in oil.

Some external leaks are sneaky, like bottom of coolant reserve container. With coolant topped off, try idle speed and parked over cardboard so you can easily see drips.

You can rent/borrow coolant tester from autoparts store. Pump system up to 20 PSI and let sit for 30 minutes, engine off. Should find leak, or it is going in the oil.

If coolant is going into cylinders the norm is lots of white exhaust smoke.

 

Thanks, Guys. She's all shut down in the garage until I get back. The oil is clean, so that's a good sign. Looks like I'll be doing a pressure test this weekend.