We love Grand Lake and usually stay at USFS campground Stillwater a little south of town. The Forest service recently built a new campground a couple miles inside the park entrance at Grand lake. It's in an area where the pine beetle killed all the trees BUT even at 29' I doubt you will find a site big enough to fit in as they put rocks at the back of the sites for no reason other than to limit the length of the sites. The bathhouse for the tent area is over 1/4 mile away from it in the middle of the RV section. Unbelievable how stupid the layout is in this day and age.
I agree with the above post that with a little education you should be able to go two or three days before the gray tank is full.