steam coming out of the heater vents

[72 cheyenne pdx]

Hello All,
I went camping over the weekend, and hadn't started up my truck in a few months. It started right up and after giving her a few minutes of running, I kicked on the heater. I started on my trip and about 5 minutes going down the road, white smoke started pouring out of the defrost vents as well as the heater vents. I pulled over and inspected the enigine compartment, but no smoke in there. I started the truck again and the smoke came back. I put my hand over a vent and realized it was condesation, so figured it was water. It didn't smell like burnt wires/oil/or coolant. After about 15-20 minutes the smoke/steam disappeared and everything seemed fine. My question is, do you think water is getting into the heater hoses? I checked the radiator the next morning and it was full, so I don't think it is coming from there. Any suggestions on where it may be coming from?

Thanks for all the help.

SEA