I had the same problem----------
RIP, you must have received your RV during the winter/ snow season. I had the same problem on my 2011 fifth wheel for I picked it up in January from RVWholsalers in Lake View Ohio. There was 6" of snow on the ground at the time I picked, the unit had been towed from the factory in Indiana
also with snow on the road. The salt that is applied to remove the snow is what is causing all the rust and deterioration.
I can see even the brass fittings are green from the salt and the screws have began to deteriorate. After returning to Florida from picking up my fifth wheel in January, I set out with a good pressure washing of all the under carriage. I even removed the wheels on each side so I could remove as much salt as possible. After the under carriage dried for a couple days.
As mentioned in a early post as using A chemical by the name OSPHO to treat the rusty areas. I used a same chemical by another name called "PREP & TECH" by KLEAN STRIP (phosphoric chemical) from Lowe's or Home Depot. It is the same chemical by the name of OSPHO. Just a little cheaper. I applied the chemical with a pump up sprayer and a quart plastic spray bottle along with a brush. This is a sort of a time consuming job, but it works. Follow the directions on the chemical container. It states it removes rust from metal surfaces & inhibits flash rusting. It also insures good paint adhesion. Again just follow the directions about application. I used a good black paint by RUST-O-LIEUM to paint all the surfaces. I purchased new screws, replaced all the rusty screws (lots of them). On the box tube of the frame near the wheels, I sprayed foam (INSULATION TYPE) into the ends of the open box framing. After drying of the foam, I used a hack saw blade to remove the excess foam that bulges out of the box tubes (FRAME).
I spent of lot of time a my back with a mechanics creeper to accomplish the task of the work to be done. Over all about a week!
As stated by the X coast Guard fellow, he can tell you about rust. Well I can too after 28 years in the Navy on lot of old WWII type ships.
Sid, Lois & HRH MISSY Terrible 2s SHIH TZU
2011 ROCKWOOD 8285 WS
2014 F-250 KING RANCH PS/SD/6R140/BFT
BOLIY PRO 3600SI GEN
DAYS CAMPED 2016 57 DAYS