As soon as we stopped and got out, we could see what was wrong: it looked like the van was on fire! It wasn't smoke, though, but steam, venting out from under the van. As we sat there, the steam died away, and it could be more obviously seen to be engine coolant (you could feel the glycol in the condensation on the rear windows where it had blown back under and onto the rear of the van). Under the conditions, Lisa couldn't determine exactly what was wrong, but we obviously had something that was venting coolant.
I called AAA and asked for a tow back to Fernley. We got back in the van and closed the doors. It was cold and we obviously couldn't run the engine for heat. Lisa already had on two coats and she me got my heavy overcoat vest that we bought in Winnemucca on last spring's trip to Spokane out of the back, and we sat, waiting for the tow.
A few minutes later, a state trooper showed up, saying that a fire had been reported here. We explained that it wasn't fire, but steam, and it was stopped now and AAA had been called, and he said okay and left. A little while after that, someone else (I think he said he was a volunteer with the fire department) showed up also saying that there was a report of a fire. We again said, no, our van was not on fire, but the radiator had overheated and the steam probably had looked like a fire to people passing on the freeway. He called the fire department and called off a fire engine that apparently had been dispatched. That's good, because we were in Washoe county, not Lyon, and the nearest fire engine might have had to come clear from Sparks.
There isn't much traffic at exit 40 (Painted Rock Road), and we had parked in such a way as to not block traffic. Besides the two vehicles mentioned above, only three others came by in the 45-60 minutes we were there, one of which stopped to ask if we needed help, which we did not at that point.
Eventually, a tow truck from Hanneman Service, with the Boss Himself at the wheel, showed up. He's a good fellow, likes us, and even knows our house a bit. With no trouble at all, he got us hitched up and towed us back home, where we parked the minivan outside the garage. The warm cab of his tow truck was very welcome in the below-freezing temperatures.
Lisa's first guess is that a hose from the heater core has failed. If so, that's a relatively easy fix. There are unfortunately a number of other things that could have happened, all more serious. Lisa says that she'll get out there and look at things when she can, although with the cold that might not be immediately. Fortunately, we have other working vehicles, including the Rolling Stone and the Big Orange Van.