TT...careful how you take some folks advice (including my own). Just because someone has something to sell you doesn't mean they know all about everything.
For example: Towing capability of TVs are NOT "marketing". It's reality from several points of view including liability/safety issues...not simply the capacity to drag a certain amount of weight around.
As for the differences between the 20' FC and the 22' Sport, I can tell you a couple things as I have a Sport and our close friends have the FC.
The FC is heavier not because of the framework. They both have the same quality box-frames. The difference is in the FC heavy, pretty cabinetry and accessories. The Sport is a less-expensive, lighter-weight interior. (The LP bottles mentioned only amount to a very few pounds difference, in fact, my Sport has 30# bottles and I'm pretty sure you can add, so I'll not try to B.S. you on that matter.)
But a BIG difference is in "useability" in my opinion. While our friend's FC is a beautiful thing to see inside... it has some serious drawbacks. For example, their emergency exit is a farce. It requires one to climb over a sharp-edged formica cabinet top, and squeeze over a tall double-sink fixture....something only a small child could squeeze past. (Meanwhile, the bed is closer to the main door, and any kitchen fire is going to be directly adjacent to that useless emergency-exit. (Later models slightly improved the matter by moving it above the mid-cabin dinette...but again, why would anyone use that instead of simply walking out the much more convenient and closer main door?)
Also, the FC has the dinette directly opposite the television...which means one has to crane the neck and stare sharply UP to view television,...hard on the neck... while the head of the bed is oriented AWAY from the television, so one cannot watch it while lying down in bed unless one wishes to move the head to the foot of the bed...in which case the reading lights are at your FEET!
Topping that off.... there is no place to "lounge" on a sofa in the FC. (There is no sofa...such as the Sport has with it's much roomier dinette/lounge.)
Also, the bathroom/toilet ...is directly adjacent to the galley (along with it's unique smells) and the bath door smacks anyone working in the kitchen in the fanny!
And, oh yeah, two axles and four tires on a longer AS are not necessarily better either. Notice that right-away many will tell you that you "must" buy larger wheels and tires, presumably for safety, ...and while some will also tell you that even single axle 20'FCs must also do that... it's simply not true. The single axle Bambi's do not suffer the tire failure rates that double axles do because the tires are not constantly struggling against each other in turns trying to tear their sidewalls apart. Bambis do just fine with original 15" tires when properly maintained, and that saves trouble AND money (not to mention making towing and maneuvering much easier as well.)
Are you sure you prefer the FC floorplan over (IMO) the much roomier and better organized 22' Sport FB with the much lighter towing weights?
Did I mention that in many states the weight of the 20' FC also require that they must receive an official state inspection and tax...ANNUALLY.... while the less-than-4K Sport does not?