I would never pick the 20" rims! Look at the sidewall of the 18" rims (lots of space between the rim and ground) and the 20" has very little room between the rim and ground.
Sure the 20" rims helps the truck drive like it is on rails, and handling in a corner is great. However it also has a rigid tire, not the 4" of space between the bottom of the rim and the roadway, so you are more likely to feel all the potholes much better. No thank!
There would be no factory installed stabilizer bar on the SRW F-350 without the camper package. And I guess no 'camper certification' either.
You can install the stabilizer bar aftermarket, and probably get a better one anyway. It might include some camper wiring. I know that mine had a relay to provide +12 battery power when the engine was running, as well as reverse lights to the camper wiring. This would be handy with a fifth wheel. I don't know if that is included in part of the trailer towing package though.
As for the slightly heavier springs, I would consider it a dis-advantage. I would rather have the softer factory spec springs, as the ride will be a little softer, and more give in the springs.
Have fun camping!
Fred!