The only place I've ever heard of it is at Fishing Bridge in Yellowstone. I'm not sure I'd WANT to use a tent trailer in grizzly country (we traveled with a Coleman Shenadoah for 13 years), but the other Yellowstone campgrounds allow them. Anyhow, the website says "Fishing Bridge RV Park is the only campground offering water, sewer, and electrical hookups-50 amp service; it is for hard-sided vehicles only (no tents or tent-trailers are allowed)."
Otherwise, we used our tent trailer in quite a few National Park campgrounds. Unless you are in low desert in the summer, go for it!