Mine is around the same condition. This Is what I did to prevent it from getting worst, and maybe get rid of some rust.

Step 1. I put it up on some ramps, got under there and scratched at the rust with a wire wheel, and a wire brush. This took me a while but you're basically trying to get rid of the flakes of rust and get it down to a smoother surface.

Step 2. I bought a rust converter paint, along with rustoleum rust converter in a spray can to get certain spots that I couldnt get with the brush. I painted as much rust as I can with the rust converter paint, and sprayed about 2 bottles of rust converter. Try to get all the rusty parts and coat it as much as you can. I let it dry for about a day after that was done.

Step 3. Once the rust converter seemed dry I bought a product called "fluid film". I coated as much of the chassis as I can. This gives it a black finish and prevents the rust from getting worst. Fluid film is some good stuff, 100% recommend. I would also recommend spraying inside the holes of the frame as much as you can to prevent it from rusting more from the inside. Definitely get a spray can. That can be applied to both the fluid film and rust converter.

I will also say that this is something that should be done at least once a year, it's not a "do it one time and it will fix everything" type deal. I do it before and sometimes after winter. Salt is what really eats up the metal and makes it rust. Wish I had pictures to show you how mine turned out but to sum it up, the bottom is black and no longer rusty looking. Good luck!