There are a great many parks in Colorado - especially in various mountain regions - which offer monthly or seasonal rates. Plenty of room for a coach your size.
However, you are not going to get a monthly rate equal to a Passport America 50% off discount.
It would be more helpful if (1) your post was on the RV Parks/ Campgrounds section of this forum, and (2) you gave us an idea of where in Colorado you are interested in camping.
This summer I have friends with motor coaches as large as your staying for a minimum of a month in the Estes Park (Paradise on the River), Gunnison (Mesa Campground) and Dolores (Priest Gulch RV) areas.
EDIT -
I called and reached two of my friends - they are paying about $800 per month for their stays in CO.