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I'm sorry to hear about your situation. First of all, regarding a "right of rescission" (a period of time that you can rescind / cancel a sale after the sale has occurred), I'm sorry to say that there's no such right in any state for the purchase of a motor vehicle / motor home (new or used). It's a common misconception that there's a 3-day "cooling off period", but that law is very specific in its applicability. That is, only certain transactions (such as door to door sales, telephone sales, and sales that occur off of the seller's location). But there is not a right of rescission generally speaking in the law. Now there could be such a right in the contract that you signed. I've seen contracts where there has been such a right, although that's still very rare.
As far as the financing is concerned, that's certainly one way that they do it. That is, because they're buying the vehicle from you with the intent to sell it on, it's not worth as much "encumbered" by a lien as it would be free and clear. Now they could buy the vehicle from you and hope that you pay off the loan on the first one (which I have no doubt that you would do) but they're not going to be making such a bet. Rather, they're going to buy the vehicle from you and pay off the remaining debt on the vehicle so that they can get it free and clear of any lien on that, and then turn around and try to sell it on. Again, it's not worth as much to them with a lien already on it, which is why they need to pay it off. After they pay it off on your behalf, that has to be offset by a debt owed by you. You still owed that amount on the original. It's not as though it went away. And the trade in amount would have been that amount regardless if you owned it outright or owed as much as you did on it. You'd get that same credit for the trade-in, but then they would add in what they have to put forward for the debt payoff. I don't see anything illegal or even deceptive in their actions.
Now it's possible that if they represented that you would not have this debt added onto the final amount, or somehow insinuated that the trade-in was "net" (after paying off the debt) then you could have a case against them, but to be successful you'd need this in writing. You would need some sort of assertion that said that they would pay off the debt and that AFTER that point you'd have the credit applied to the new motorhome. In the absence of such a written representation, I don't think that you'd have any recourse.
I know this is probably not what you wanted to hear, but it is the law. I hope that clears things up anyway. If you have any other questions, please let me know. If not, and you have not yet, please rate my answer AND press the "submit" button, if applicable. Please note that I don't get any credit for the time and effort that I spent on this answer unless and until you rate it positively (good or better). Thank you, ***** ***** luck to you!