what grade of aluminum for Aerolite frame?

[Phoenix51]

I'm looking to have additional cross-members welded into the floor framing on my just-acquired 1997 Aerolite 21RB. Any idea what grade of aluminum is used in these frames? What grade of aluminum rectangular tube should be used for the additional cross-members?

I'm replacing the entire floor as it was sopping wet and rotted. The floor sagged and I just found that a cross-member was broken. I'm stripping the floor down to the frame, and am removing/discarding the underside metal exterior skin as I can't believe that to be a great design idea, once water gets in how would it ever get out.

Any trick to removing the floor-to-ceiling wall panels without damaging the ceiling?

[Phoenix51]

I picked up 6061 rectangular and square tubing for the additional cross-members. I managed to remove the old floor without removing the floor to ceiling walls, so things are moving along. I've decided to retain the bottom metal sheath.