The cheapest campsite at the Fort is about $55 in the low/off season.
A popup campers is $75/night at its cheapest.
While room costs-to-room costs is slightly more than a Value resort, the camper has a kitchen and a fridge so you can save serious bling by cooking and eating meals at the camper or packing a lunch into the parks and use counter service or table service dining occasionally or rarely. The popup rentals have A/C, folding chairs, etc all you need for a stay.
If you're not regular tent campers, sleeping on the ground is not a newbie's idea of a vacation, I would say.
The Motorhome and you in the rental popup could request loop 1100 and the cabin folks can request loop 2100. If the three of you link reservations the Fort will try to assign you as near to each other as possible. Some campsites in 1100 are a 10 second walk from some of the 2100 cabins. I would think staying apart from everyone else would be a bummer. (1100 is not a cheap loop, though)
Bama Ed
PS - I think if you have more than 10 folks between the three parties you can use Disney's Grand Gathering reservation service.
PPS - if the Motorhome will be at the Fort, you can put a tent on a site with a motor home. You can rent a tent from Disney and cots too (I don't think the combined would cost near $100) and since you don't have to pay for a site, that saves money. Just bring pillows, blankets, sheets from home for the cots.