Amanda:
RV Loop 1100 and Cabin Loop 2100 at
Fort Wilderness have a row of cabins and campsites that back up to each other, but my understanding is that your request isn't unusual and the proximity of these sites is popular. So get your request in ASAP. Keep in mind, also, that because Disney doesn't guarantee a site request, you might be a good distance from your RV loop, but if there's an opening closer when you check-in due to a last-minute cancellation, they are more than willing to accommodate your request if they can.
My recommendation is to make it clear on your reservation that you are driving down in your RV and will be using your RV during your cabin stay and would like it in a spot that is as close as possible, preferably Loop 1100. Another option is to ask for a cabin with a large parking pad that will accommodate your RV's length (and give the length). If it's not a Class A, you may be able to squeeze it in. If none of these options work, you can always park your RV in the overflow lot out by the Outpost Bus parking and access it.
Wishing y'all a magical vacation!
Whitney