Know nothing about Grand Manner boats but the steamer in question at Little Big Horn was the Far West and I have a primed but not yet painted model of it from OldGlory25s shipyards - in 15 mm. I'm sure you can google images of the boat.
And, yes, they were quite common on the rivers of the west and the main means of moving large quantities of freight where the railroads did not run - with a navigable river, of course. Challenge was some rivers only had enough water for part of the year. The Far West ascent for Big Horn was one of the highest (the highest?) ascent of the river up to that point. The Mississippi must have had hundreds of boats at this time, maybe thousands if you count all the tributary rivers and especially the Ohio River.
Not, however, that common in Plains Wars that I'm aware of.
We actually have a restored steamer of the era converted to a floating hotel on the local river near where I live - the Sacramento River, the boat being the Delta King (formerly of the Mississippi if memory serves).
I'm on the lookout for a riverboat to use as a gambling institution as part of my Old West/Plains Wars/Apache Wars collection. One of the few items I'm still looking to add to my collection that might be available - the others things I want not yet made that I know of, like a horse drawn hears in 15 mm and a chain gang from the local territorial prison with suitable guards, and those Chinese for the laundry and the joss house - and maybe a damsel in distress, or would that be undress?, an asian soiled dove of three.