If you have not taken possession of the rig or the title has not been put in your name the dealer can still sale it as new. As quickly as you can tell the dealer your truck will not handle the load. Tell them you will still purchase from him but you need to find a suitable trailer on his lot that will match your truck. An ethical dealer would work with you. Explain your newness to rving and the reliance placed on the salesman. Talk to the owner not a salesman or finance guy or manager. Call the bank and tell them to hold up processing the loan and why. Don't sign anymore papers.