A lot depends on what sort of CG you're talking about. Many (but not all) commercial CG have tent spots, but as pointed out, there's limited parking availability for a trailer. There's also the "hybrid" type CG, such as Estes Park at Mary's Lake, that have some no hookup sites big enough for trailers along with the tent sites they provide, which are limited to tent campers.
In RMNP, as well as the USFS CG that I'm familiar with in Colorado, you'll find that most of the tent sites are really not adequate for trailers, as previously mentioned, the parking areas are for vehicles, the tent pads are away from the parking area. When one goes on line to make reservations for the RMNP or USFS sites (Recreation.gov) you can of course not put in your trailer length, and get a list of all the sites, but you're doing yourself and others a disservice if you try to reserve a spot that your trailer clearly won't fit.
As with anything, there are always exceptions and YMMV. If you look at various USFS CG on Google Earth, you'll see that for the most part, there's space between camp sites and in many cases, there's a lot of space in between. Yes, there are also some sites where they are closer together, and these are usually preferred by folks that are camping in a group. Also, on recreation.gov, many times there are pictures of the camp sites, so one can see what's provided, before making a reservation.
