Aside from the CA restrictions that Tiger cites, I haven't been affected by any length restrictions and I've traveled pretty much everywhere in the US. I did the North end of PCH 1 before I knew about the restrictions. I had a car in tow. I made it through OK but I wouldn't do it again. Didn't get a ticket, either. Lucky I guess.
One other road I know of is in Glacier Nat'l park. Best bet is get a copy of
MOUNTAIN DIRECTORY WEST (East is pretty flat). That way there are no surprises. If you have a car with you, you really won't miss much. Stay at a nearby campground and go sight seeing in your car.
I run into more weight restrictions on back road bridges than anything else but I've always had enough warning that I wasn't inconvenienced or I did it anyway because my weight was so closed and I figured there must be some tolerance involved (about 50,000 lbs here). (took a chance, no consequences. YMMV)
Some campgrounds have length restrictions but not as many as you may think. Of the ones I have run into, some are not enforced (Coast to Coast/Encore group if you contact the campground directly and don't go through their 800#), some will walk you through the park and try to accommodate you and sometimes it's just a matter of going down the road a few miles to find someone who can accommodate you. I've never been inconvenienced that much.
