The more I think about this, the more I think Disney will eventually close this loophole. The whole point of making staying onsite more advantageous is to get those dollars spent onsite. Booking a one-night campsite will eventually cause them to lose plenty of money because it makes it advantageous to book a one-night campsite, rather than to stay onsite.
Personally, I wouldn't attempt this because at any time, Disney could require that the campsite be actually checked into and used, and at any time if you are not there, they could probably eventually drop any Fastpasses associated with that stay.
As with many things FP+ related, there are loopholes that are advantageous until they get closed. Remember people using old KTTW cards from previous trips? They figured that one out and fixed it. They'll fix this one too, because they want people to stay onsite, not to book a campsite for one night.
The bummer is, we don't camp at Disney because we're cheaping out or can't afford nicer accommodations. We do it because we love it. We stay in all levels of resorts, but camping at Disney is a really, really lovely experience.
But people do what they have to do within the limits that a system sets up for them. And to be honest, I do not know how common this practice really is. Campsites fill up during the weekends during nice weather. Weekend campers go and sometimes don't even visit the parks. Fort Wilderness is a REALLY nice campground. Other than weekends, I can find sites for pretty much any weekday in December.
It is what it is. I'll keep my fingers crossed we can pull off a longer camping trip this December. If not, we'll camp five days and then relax at VWL or WL for a few days.
But again, if anyone wants to cancel their weekend December campsite for me, I'm totally okay with that!

(On the off chance that someone does do that, PM me so we can coordinate and I can pick it up!)
VP