Daniel I think folks are getting hung up on the thinness of Simpson Products - even though they do make some thicker specialty products also. Just find some galvanized angle iron of appropriate thickness such as 3/16. Tap your bolt holes and then clean it properly - rinse in hot water and let it dry - don't clean it with something like mineral spirits that will leave a film - diluted ammonia works well. Prime it with a galvanize primer and paint it. It should outlast anything your doing to the trailer. Don't use Stainless fasteners to bolt it in place.
http://www.sherwin-williams.com/pro/problem/problems/peeling_galvanized/index.jsp
When Airstream replaced the tank hanger on our 71 they used 1/4 aluminum but coated the contact areas where it will touch steel. A local Airstream mechanic said he uses two layers of duck tape as the buffer on the contact points. Only the rear was replaced on ours. The forward bracket is original steel and can't be any thicker than 1/8 - it's rusted badly and will be replaced with aluminum also.
If you can find angle in Galvanneal it is much easier to prep and holds paint much better. I had a new battery stand made from it and used automotive primer and a couple coats of Rustoleum.
Galvannealed
(see also Jet-Coat, Paintlok)
Is the result from the combined process of galvanizing and annealing the steel. The galvanization is made through the hot-dipping (Hot-dip galvanizing) process and gives a very fine grayish matte finish. Galvanneal does not flake off its galvanized coating when formed, stamped, and bent. The very fine matte finish acts like a primer, easily adheres to paint, and is very rust proof; only white to dark grey marks appear if it comes in contact with water. Galvanneal sheets offers good paintability, weldability, corrosion resistance, and formability. It is extensively used in the automotive, signage, electric equipment, and other industries requiring a metal with good paintability and long reliable service life.
If you have any scrap yards in the area many will sell various types of angle they have taken in as scrap. The one locally has racks and racks of bar and angle stock - and it's cheap.
Good luck with your project.