By "Bend" I assume you mean Bend, OR. If so, I'm guessing this is from exposure to salt water, the ocean air and road salt. After wire brushing you should be able to tell if this is just surface rust or something more serious. Minor pitting and flaking is OK. Hopefully, if it's more serious, only a few, easily replaceable parts are affected. Otherwise, I'm with Steve and Chris. I'd use POR-15 on the more seriously corroded parts.
You can search the Sportsmobile members list for owners in your area who may be able to give you an opinion via private message. You can search it here:
https://www.sportsmobileforum.com/forums/members/list/
My E350 came to me from the midwest. Here's what I discovered under my cab's floor mat:
https://www.sportsmobileforum.com/fo...diy-27589.html
I also had some rocker panel rust and some underbody rust that wasn't as significant as your's (I left both untreated). I suspect my floor pan rust was so bad due to the wet fiber on the underside of the floor mat never drying out. I'm in the Sacramento CA area so I'm not worried about future rusting.