The only places in the Denver area that I know anything about are state parks, St. Vrain just east of Longmont, Chatfield southwest of town and Cherry Creek southeast, near Centennial airport. All are decent, but weekends and holidays are filled up almost as soon as reservations are allowed, six months in advance. Weekdays (Sun. nite to Thu nite) there are often openings.
Looking in Allstays Camp & RV there are several commercial campgrounds around but some don't seem great. There is the Denver Meadows RV Park in Aurora (east) which might be worth looking into, and perhaps the Flying Saucer RV Park in Englewood (south), as well as the Dakota Ridge RV Park in Golden (west) which has a max length of 45 ft. There's also Barr Lake RV Park in the Brighton area (northeast), near Barr Lake.
I know absolutely nothing about these commercial parks, but the ones I listed are ones that the Allstays listing looks OK, so I would check on them if I were needing a place here.