We used Mountain View RV
(a PA CG) a little west of Columbia Falls last summer(mid May) for 8 days and for 10 days the summer before last(July). It's a nice, reasonable park($20-$24@night) and is a little closer to GNP. We found prices were higher in Whitefish, I guess because of the ski resort attitude
, but mainly we found CF to be more centrally located. The drive around Dead Horse Reservoir is nice...lots of dirt roads to explore. Saw some waterfalls and an occasional moose.
Of course, you probably already know not to miss 'Going to the Sun' road inside GNP...one of the most spectacular roads we've ever driven. We drove into St. Mary for ice cream and on into Many Glacier where we saw our first ever grizzly. Lots to do and see in the whole area. Could spend two weeks there...easily.
BTW, this past summer the road to the sun was not open yet(May), so a later trip (in July) will (almost) assure a drive on that road. Never mind, I see the Aug. date now.