I go to Dillon every summer to visit my cousin for a couple of weeks and fish. Done a lot of traveling in the area you are talking about in my camper. I usually stay in USFS campgrounds. I do remember one boondocking spot on the Flathead River, where Glacier NP is across the river. Here is a Google
map. .
I keep a very good website of my travels (see signature link). If you go to the Campsite Map page and look at all the separate year pages, you'll see every place I've camped. You might find a spot from there, although most are USFS campgrounds. However, USFS boondocking sites in the general area of the campgrounds are likely.
The boondocking spot I mentioned above was on 8/5/09 and a picture is below. The mountain is in Glacier NP and the river is at the base of the mountain. My log says the road was very washboarded. Also not particularly private, since boaters/rafters launch there.
Have fun. I'll be back in that area in August and I'll look for your Casita. :)
