in the panhandle here are a couple suggestions...
sam owen and Farragut parks on pend orielle lake
dwarshak reservoir - dent acres campground.
mccroskey state park - dispersed camping.
Wallace Idaho.
priest lake. I would go there on weekdays as it gets very busy on weekends.
lots of smaller campgrounds going up highway 12 from Orofino to lolo pass.
hells gate state park just outside Lewiston - nice starting point for the snake river gorge.
that is some of my suggestions for the panhandle section.
you said hwy 20...are you talking about hwy 20 through Oregon into boise Idaho? still... if you make a trip up to northern Idaho those are some of my suggestions for you.