You might consider writing a demand letter. Such a letter would "demand" a certain result, likely either the return of the camper or paying the money. Additionally, you would provide a clear deadline for a response. For example, you might state, "If you fail to respond by October 15, 2011, I will have no option but to pursue every legal remedy available."
Failing this, another legal option would be to file a civil lawsuit. You might consider filing a lawsuit based on his breach of contract in failing to make the payments as scheduled.
While you could potentially handle these matters on your own, it would be prudent to consider retaining an attorney to assist. Even as to the demand letter, this would be much more compelling coming from an attorney.
Please let me know if anything requires clarification.
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