Legal Question in Real Estate Law in Indiana

It is hard to tell from your question, but if you agreed to pay $2000 for a car, had paid all but $100 as agreed, and were then told that the other person was just going to take the car back and keep the $1800 as "rent", would you think that was fair, and would you think of that person as a friend? I would go to small claims and sue for the rest of the money if needed.

You cannot just unilaterally decide to change the terms of the original agreement. Your rights and her rights in the trailer have to be decided. You can't just keep the money that she has paid and call it "rent". Try to work it out first--tell your friend that you have no alternative but to go to court if matters are not resolved satisfactorily. IF all else fails, either consult with an attorney or go to small claims court.