Our 2951 has a large open area (about 8" deep) behind the rear mounted fuel tank, just below the garage floor.
Fabricated this under-chassis platform to take advantage of this open space. Ended up with a pretty significant 78" long x 30" wide x 18" high open storage space. It hangs about 9-10" below the frame rail---about the same road clearance as the bottom of the axle tubes. We try to be careful on dips, but so far no issues after 2,000 miles. If it does become an issue in the future, it's easy to adjust higher.
The platform is held up by 6 steel straps which are bolted to two 1 1/2" square steel tubes that are resting across the inside lip of each of the 2951's I-beams. Use a dual-pulley system to raise/lower either end of the platform to store/access larger items.
Granted, not the most convenient way to store items you need daily. However, we store our 3-ton floor jack, 42 gal. waste tote, and 3000w backup inverter generator here. Items we don't use regularly, but become priceless when we do need them. Nice not to have to store this stuff in the bed of the truck, in plain sight, exposed to the elements---especially on our longer, cross-country trips.