Saturday, September 23, 2023


Ask Dave: Why does RV wall have rust stain, but no apparent leak?

Dear Dave,
Any idea what is causing this rust-like stain running down the driver side of my Class C RV? It’s coming from behind the molding at the top of the photo, but there’s nothing wet inside. —Dennis, 2017 Thor Freedom Elite on Ford E-350

Dear Dennis,
This panel is commonly referred to as the “wing wall” of a Class C unit. It is the transition point of the cab from Ford and the body built by the RV manufacturer, in this case Thor.

Typically this RV wing wall panel is just plywood with an outer fiberglass material laminated to it and an inner luan sheet with decorative wallpaper similar to the inside walls. It is connected to the sidewall with “L”-shaped steel brackets and bolted to the cab’s flange. The flat “floor” of the overhead bunk can either be plywood, or some RV manufacturers build an insulated floor with aluminum framework. This floor is secured to the sidewall framework that wraps around the curve of the bunk with screws.

Moisture in the RV wing wall

Since you have rust coming down the seam at the wing wall, there is moisture penetrating somewhere above and leaking out at the wing wall to the overhead bunk floor. The challenge with moisture leaks is it is hard to find where the moisture actually enters the rig as it seldom exits anywhere close.

I would start by checking the roof to sidewall and overhead bunk seam. You could have a leak that starts up there and follows the hollow tubular framework. I’m not sure if Thor uses steel or aluminum framework. However, both are hollow and provide a raceway for moisture. The rust could be coming from a steel framework or even the steel screws. Also check the rounded seam of the overhead bunk to the sidewall to make sure your sealants are doing just that, sealing.

One other area to inspect is the actual cab to the overhead bunk floor. With road vibration and the weight of the overhead bunk, it’s not uncommon to have moisture penetrate between the cab and floor—especially during a driving rain—and then migrate to the path of least resistance.

 You might also enjoy this from Dave 

Ask Dave: Why can’t the RV industry conquer the issue of leaks?

Dear Dave,
I read everything I can about RVs and maintenance. One thing that is constant – leaks. Why is it that the RV industry cannot conquer this problem? I read all the time why it happens, but no one seems to have the right way to build an RV so it doesn’t happen. The exception is the two-piece fiberglass units. I’m curious why this consistent problem can’t be conquered. Thank you for your response. —Sharon

Read Dave’s response.

From Thor: Class C Wing Wall – Remove and Replace

Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”

Read more from Dave here


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Dave Solberg
Dave Solberg
Dave Solberg is a leading expert in the RV industry and author of the “RV Handbook” as well as the Managing Editor of the RV Repair Club. He has been in the RV Industry since 1983 and conducts over 15 seminars at RV shows throughout the country.


  1. Also don’t forget the window over the cab, I have had many forward windows leak from failed sealants and water migrates back as Dave stated…


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