hopefully you have somewhere sheltered to work on it. if not id start with that. next strip out all the appliances and plumbing and everything useful. then demo the rotted box off the frame. next service the frame, this is a good time to clean up any rust and weld in any supports and patches. then start building the box you want in the configuration you want. reinstall the appliances electrical and plumbing as you build. then put a lid on it and cover it.
been there, I wouldn't do it again and I did ok with mine (bought for 2k, spent 1100) sold for 1800 5 years later.
don't expect to be using it this season unless you have WAY more time than I do to work on it.