You probably are right that a possible source of the fire was an improperly lubed front wheel. I had our DP lubed and immediately proceeded over the Rockies and down from the Eisenhower Tunnel. As I did so (for probably the 10th time), as usual I relied on the exhaust brake and only tapped the brakes occasionally when speed built up. Unfortunately the speed built up more than it should have and suddenly I smelled brakes, then had smoke coming in through the AC. Knowing the road I was able to negotiate to the bottom since I was almost there and did not have to take an escape ramp.
The cause of the problem? When I had a knowledgable mechanic get to my rig he had a very difficult time removing the right front wheel to see what happened. When he finally was able to do so we found that only 2 of 4 Zerk fittings had new grease. One of the other 2, the one that controlled the S cam, were dirty and had not been serviced. The result was that the S cam locked up resulting in the brakes on that wheel being locked on and causing the brake shoes to crystalize, burn and become useless.
The left wheel was fine. I now check after lubrications to be sure that all fittings were serviced.
Incidentally, after seeing a demonstration of various fire extinguishers, we have gone with foam extinguishers. One of their advantages is to keep fires from flaring up again after being intially knocked down. The contents also do not settle as the regular ones do.