There are several options depending on what route you are taking to Montana. If you go via Salmon and US 93, there are BLM and FS campgrounds along US 93 that will fit your 5th. Just north of Salmon is a BLM campground right on the Salmon River - Tower Rock IIRC. Further north on US 93 are several FS campgrounds, but they may only have one or two spots that fit your trailer well.
If you are going over to Eastern Idaho and up I-15 to Montana, there are a few good spots. One is the North Bingham County Park
http://www.co.bingham.id.us/parks_recreation/north_bingham_park.html that is right on the Snake River - take I-15 Exit 108 East for 1 mile. Other options closer to the border include Stoddard Creek FS campground just north of Spencer - lots of boondocking in the area also.
Finally, if you are going up to Yellowstone via US 20, there are several FS campgrounds in the Island Park area, but they are typically busy, even during the week. Some take reservations and some do not.
Hope this helps...
