Another vote for the Watchman CG but you will need reservations for sure and the earlier you make them {up to 6 months I think} the better. We have been several times, last was 2 years ago and the $18 sites {{$9 with a Geezer card} all had 30 amp service, as noted there is a decent two-lane dump station. When you stay at Watchman it is a short walk to the visitors center/tram station which is the best and often the only way to see the park.
The weather can change quickly but May should be gorgeous. Enjoy!
:C