Evaporust and naval jelly are opposites and mechanical removal before using them is far better, if the rust is heavy or you have peeling paint.
Naval jelly is a jelly and is an acid. Evaporust is not an acid and it doesn't harm other substances or cause pitting to good metal, plus it is a liquid that is hard to keep wet on a large part.
If there is very rough rust or flakes of rust, paint, or dirt, do a quick clean up job with a wire brush, flap wheel, sandpaper, scraper, or whatever you have. You must remove oil and grease before using chemical rust removers. Even a pressure washer can help, depending on how bad it is. Media blasting is the best, but for smaller areas, naval jelly can work. It has to be thick enough to keep the surface wet from the jelly. The jelly is just a carrier for the acid. I use pool acid to dip my cnc plasma cut parts to remove mill scale, dross, and rust. The directions on evaporust say to use a paper towel or cloth soaked in the product to keep a wet surface.
After using the rust remover, you will need to rinse and dry the frame. There are also rust converters for lighter rust or areas you don't get all the rust removed. Good primers can encapsulate the rust and stop it if it isn't feasible to remove it. If moisture can't get to the metal then it can't rust.
The ultimate job would be to remove all the rust but it sometimes isn't practical or worth the trouble. How bad is your trailer and how good do you need the paint job to be?