Ron:

A 40 degree night shouldn't have been a problem.

Have you had your compression checked lately? At 6500 feet elevation the air is considerably thinner than down in the valley. A diesel that starts just fine in Sacramento or the Bay Area will continue to do so with reduced compression.
At 6500 feet (or higher) the molecules are not very close together and it becomes harder to compress them enough to get combustion. Having said that, the 40 degree night was a contributing factor in that the oil was thicker and the battery didn't have quite the same energy that it might at a higher temperature.

We're in Arizona at 3500 feet, but have a cabin in Alpine, AZ at 8000 feet. My '82 300D Turbo would start and run fine at the lower elevation and would continue to run OK at 8000 feet. However, starting it with anything short of a very warm engine was nearly impossible, even with a block heater. A rebuild solved the problem. The new compression causes it to start within the first revolution, even without the block heater. (We normally only use the block heater in the dead of winter when we can get sub-zero temperatures at Alpine.

Irg is correct regarding the use of #1 diesel. Try to either get that or else buy a "winterizing" additive and add it before you go up.

Wes

P.S. Lived in Nevada City for over 30 years and had no problems starting there or at Tahoe with a different diesel MB.