Rust is like cancer, if you don't make the time to tackle it soonish... then it will spread and be a much bigger problem. I just finished fixing the same sort of thing on mine... different than a few others have done it before. Should have a thread up in a few days about how i did it, it might be more cost effective, but requires a welder and a TON of patience.
Of course, you could just get a can or two of rust stopping primer from rustolium or the like, spray like hell, drill some holes in the bottom to allow water drainage, then click the plastic back on over it and start saving money and deal with it next summer (when you have funds) That way you can slow/stop the progression fo the body rot while not dropping massive coin and/or time.