Yet another problem which has been solved numerous times in the past. Do a "Search" using the keywords "Camco 55572" (be sure to check the "Archived Posts" box) and you will receive the answer with pictures.
Basically Montana installs the 50 Amp plug with the silver ground plate in the 12:00 o'clock position. Camco's plugs have the corresponding ground plate at the industry standard location of 8:00 o'clock. All you have to do is rotate the receptacle on the Montana 1/3 of a turn counter-clockwise.
The detailed instructions are in this posting.
Each page is informative but the instructions and pictures on page 5 will show how easy the change is.