"Best" is always subjective, depending on what your needs/wants are in a CG. That said, there are some choices.
Two commercial CG/RV Parks just outside of town on the way in to RMNP. Winding River Resort and Elk Creek CG and RV Park. Both are nice, Winding River has a a choice of no hookup, W/E hookup and FHU sites, and is a bit spread out. Sort of like camping on a ranch, it has the kind of vibe to it.
Inside Rocky, there's Timber Creek CG, which is all first come/first served sites. Though I've seen rigs your size there, it's fairly tight for bigger rigs. Tight not due to trees, as they've been cut down, just that the sites cater more to tenters and smaller rigs. But it could be doable.
South of town there's about a half dozen USFS sites that could be doable for you. Stillwater CG has some W/E sites. The others don't.
H E R E - Arapaho NF is a link that will take you to the USFS website. The ones listed for Granby are what to look for. Most of these will be able to handle bigger rigs, as I've always seen large 5ers and MHs there.
If you are OK with being a bit further out, there's Snow Mountain Ranch at the YMCA of the Rockies. Great sites there, they have FHU sites available and can handle bigger rigs.
A caveat. Depending on when you plan to go in September, some of these may be closed or closing. When looking at the USFS sites, look to their season opening/closing date. Stillwater for example, has Sept 22 listed as closing date, while Green Ridge is open until Oct 14. Also, some of these will most likely be fully reserved until the end of their season. Winding River is open until Oct 1.
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