Technically speaking, the "Cabins at Fort Wilderness" and the "Campsites at Fort Wilderness" are two different resorts. It's like trying to link a Contemporary room with a Boardwalk room. The Disney reservation system does have a method online to link nearby campsites (I don't know about nearby rooms). But to do what you ask will require the "brute force" method.
What you will have to do in this case is "overcommunicate" on both the 2100/campsite reservation and the 2200/cabin reservation (that was our favorite cabin loop way back in the day) with complete reservation numbers and names added as a NOTE on the other reservation via a phone call to the Disney reservation line.
Example: Call the Disney reservation line and tell them you want to add a note on your CAMPSITE reservation. Ask to stay "nearby" and give the cabin reservation #, arrival date, guest name, and say it's a cabin. Ditto on the CABIN reservation. Ask them to add a "nearby" note for the campsite with all the same info. And for the cabin reservation ask for it to be assigned to loop 2200 and the campsite reservation ask for it to be assigned to 2100.
This is not an uncommon occurance for the Fort Wilderness CM's who alter the computer default cabin/site assignments based on what is available on arrival day. That's why I say "overcommunicate" and give them all the info they need on both reservations to understand what you are trying to do.
As long as you do this several days before arrival, it could work. But I would suggest both cabin/site folks to check in at the Fort in person rather than via online to make sure this happens face-to-face.
Bama Ed
PS - and I take my Aliner to the Fort about once a year or so. Here was November 2022 in 100 loop.
PPS - and we do one park day paired with one non-park day AT MOST (half the time at FW during the day or at Disney Springs or doing a monorail resort crawl or lunch in the BW area with a Skyliner tour or putt-putt. We aren't big theme park folks since we go almost every year but we generally do MK each trip, Epcot every couple years, DHS less often (although did it in Nov22 because of Rise and Mickey's RR), and AK once every decade. Sometimes we only do one park day in a 5-6 day Fort stay.