If you want to go a little out of town, Samowen in Hope is very nice, near the water. Farragut SP is huge and nice. There is a swimming area but campsites are not very near it. Lots to see there and there's the Silverwood theme park not far away for the kids. Garfield Bay campground is also not far from Sandpoint. Haven't stayed there myself but it looked pretty nice, my dad had his boat moored in the marina there for a while.
I don't recommend the Hi De Ho RV park in Sandpoint itself. We stayed there for a night to restock and do laundry last spring. It worked for that purpose. People were nice and didn't have a "bad" experience but it's not scenic at all, RVs packed in there (some obviously never moving again), near highways and trains.
If you want to go a little farther north, Priest Lake is amazing. My personal favorite place to camp out of anywhere I've been. Not a bad campground on the whole lake.