How can I get the most usable space without breaking the bank?
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Look into getting to know someone who designs for ships, yachts, mobile homes, and RVs. In these there is a massive amount of storage that you would never dream of until someone showed you. My sister and her DH lived in one for years after they retired, and I never tired of seeing where she could stash stuff. From the bed itself capable of being raised for underneath storage, to roll top desk like covers over storage over doors and windows, all over the place. Just visit someone or look at RVs online and check out all the storage there.