This all depends on your idea of what a conversion will consist of. Do you want all the luxuries of home (hot water, ac, electricity ? ). All these things take time and money. Or are you happy to have a place to sleep and you will read books and use battery powered lamps?
As you have already pointed out paying rent will greatly slow this process. It will also do something else. You will get comfortable in your apartment and spend less time and money towards your goal of living on the bus. Living simply and cheaply in an unfinished conversion has a way of keeping you motivated! Just keep your goals realistic and don't spend money on thing too far in advance. Even if the deal is too good to pass up. Getting a super deal on cabinets and then not having money to insulate will stall the project. Ask me how I know :/
We live in an Amish community and I'm sure in Wisconsin you are near them too. Living simple can be done cheap without sacrificing quality of life. Just talk to the Amish
You are young...go for it now!