PCs don't have rear end sag.
Tom is correct for all but one or possibly two current day models and more earlier models. Since the 2008 E350 came out, the sag has been reduced because the 2008 E350 chassis handles a heavier load than previous model years. For current 2013 production, I think the rear end of a 2100 & 2400 on the E350 still may have a sagging problem. The recent 2350 has less a problem (maybe none) because sometime in the past few years, PC-USA improved the weight distribution by rotating and moving the fresh water tank forward.
The sagging may not exist when those particular motor homes are coming off the PC production line, but rather after the owner loads the rig with personal belongings, a full tank of fuel, full tank of propane, and a full tank of fresh water. Being that 2100s and 2400s are E350s and have the water tank over the rear bumper, the rear end starts to drop with the weight of the fresh water. It's like putting a trailer on the rear hitch with a tongue weight of 400 pounds. It's going to sag a little.
PC's made on the E350 chassis prior to 2008 have a more notable sagging problem because the chassis back then handled 1000 fewer pounds. Our 2007 2350/E350 loaded for a trip has a little sag but not enough for me to do something about unless I planned to take ferries through Alaska. Like the 2100 and 2400, our older 2350 has the fresh water tank over the rear bumper.
The most common and affordable means to eliminate a sagging rear end is the addition of rear air bags or adding one of more rear leaf springs per side. Air bags are height adjustable. The more air you put in, the higher the rear goes up. But the amount of travel is limited to around 3 to 4". At least that is what was with our first motor home.
Stabilizer bars, sway bars, and anti-sway bars are the exactly same thing going by different names. They are used to stabilize the vehicle in winds and turns. They reduce the amount of side-to-side lean. They do not affect the height of the motor home. Nor do heavy duty shocks, trac bars, or heavy duty steering stabilizers. They only improve handling, not change the height.CLICK HERE
to see the Firestone air bags I mentioned. The air is adjusted like a tire using the same type of valve system. You can make them more user friendly having an on-board compressor and controls up front to adjust the air while driving. That was the setup with our old rig. I really liked it and used it. I would adjust it less for a softer ride, more if more height or better handling was needed due to road conditions.
There are other air bag brands, but the Firestones seems to be the one of choice for affordability. This based on my observation on other RV forums.