My favorite spot for that long would be Mueller State Park just west of Colorado Springs, elevation is a little high but the sites have electric. I think you would have a hard time finding a spot with a month open though. Same would apply to Cheyenne Mountain State Park in CO springs.
In Denver you might be able to get into Chatfield State park for that long. I live very close to there, and it is nice with easy mountain access but no tree cover. Otherwise, there is a "average" RV park in Golden that could accomodate you for that long or possibly Cherry Creek State Park, further from the foothills.
2009 Georgetown 350TS (bunks)
400W solar, 440AH 6V GC2
2009 Ford Edge AWD Ltd towed
2011 Honda Fit Sport towed