The mountain men ate mostly meat because that was mostly what there was to eat. Flour or cornmeal were luxuries. When other stuff could be found, they ate that, too.

There's not a lot of forage in western forests. Berries, camas root, wild onions. The native Americans probably taught them most of what there was to gather. Mountain men often spent let's say "quality time" with native american women, who would have been experts on that.

In regard to scurvy, you can get at least a minimal amount of vitamin C from foraging, provided you aren't on board a ship in mid-ocean.