Yellowstone CG reservations is a sort of gauntlet test. We have been there many times and reservations are always difficult, how many feet,are you sure, even with a truck camper they will cross examine like .....
We have stayed once at Fishing Bridge and it is tighter than the other CG in the park. I believe that reservations are by site. You may be able to compare the park campground map, if there is one, with Bing maps and see which sites are longer and focus in on one that will work for you.
Things seem to work out.
Good luck.