I agree with your idea. re: Using adjustable Jack stands instead of under built factory scissor jacks & sideways stablizer bars. Although to me, it isn't a "poor mans" fix. It's actually "the smart man's fix". Especially when elimination of movement is needed.
If wondering, I use many 6-ton (next time, I will use 12 ton for more stability) axle stands under my long TT. They are set approx 6ft apart - down each steel frame rail. They eliminate both up/down bounce and sideways wiggle. For picture, surf: http://i178.photobucket.com/albums/w...e/100_0696.jpg
IMO - Have the RV dealer install the enclosed underbelly plastic material sheets. If you install it and encounter future problems, it might void factory warranty. If they install it and accidentally screw into a wire or water line, then they fix it (under their dime). As soon as the base warranty is completed, suggest removing the enclosed under belly material, install Contractor Grade Closed Cell @ 2 lbs approx 3.5-4.0" thick Spray Foam, then re-install the enclosed under belly materail. If wondering, my Jayco TT has spray foam on its underbelly. That Contractor Grade Spray Foam is amazing. If applied properly with sceen screen covering access holes (then spray foaming), it keeps critters out, it keeps heat / cold in (which saved energy while bone docking) and best of all, Spray Foam @ 3.5 - 4.0" thick "tightens up" floor movement. To me, all TTs/RVs must use Spray Foam insulation in their base build (but that's a topic for a different post).
Hope this helps in our research....