I have the Igloo on a Dodge Ram 2500 Diesel. I added HD Rancho shocks and airbags. The ride was harsh and the airbags seemed to intensify the sway. I then added supersprings and was able to set the shocks on a lower setting. I now have a pretty decent ride and the equivalent of a 3500 Dodge.
My Igloo is much lighter than the 12 footer. I am well beyond the cargo rating and at the limits of what works for a SRW. In my opinion there is absolutely no doubt you should have a DRW truck.
You will find that it is easy to add lots more weight than you anticipate. I have two solar panels, big AGM batteries, and a generator. Even being careful consider all of the extras: clothing, food, cooking gear, hobby equipment, tools, gas for the generator, plus a load of water, 2 propane tanks, and whatever your fellow travelers decide to bring and collect along the way.