O.O.,
Perhaps you missed the page where your personnae talked about his "...re-assignment to the Steamer FAR WEST, wherein I was told to stand night Guard Duty and make sure that the two Gatling Guns and accompanying Ammunition Waggons left with the Steamer by Mr. Custer, Geo. A., did not fall into the hands of the Enemy."
Of course, your daytime 'Duty' would no doubt have included some shipboard work, and as you were a known Painter, some touch-up brushwork on the Pilothouse would have been just the thing to keep you busy.
No doubt your assignment as a Guard would have then been deemed necessary to have you continue to stay with the two Guns on the fast trip back downstream with the news of the 'Last Stand', one of America's fastest recorded Steamer trips in the history of the Old West!
Best regards, good reading and hang on for the ride!
'Ol Gabe