I never dealt with out to state car purchase, I was under the impression that if i went to go to the DMV they wouldn't take the trade in out. Looking at the RUT form, it looks like i would have been able to use that credit anyway.
Live and learn i guess!
I think most states will work with you if you have your ducks in a row. Here is how I did it in MO:-
Bought the 2500 from Mark Dodge in late April. They did not collect any LA sales tax from me, because I financed it through a local credit union in MO.
Sold my 1500 on FBMP about 3 weeks later.
Went to my local "DMV" in St. Louis with my paperwork for the new truck, as well as a Missouri Bill of Sale for the old truck. I also had a MO Title Application filled out.
They subtracted the selling price of my 1500 that I sold privately (after buying the new 2500), and adjusted the taxable purchase price accordingly.
This is essentially the same process as a trade in, but I had the benefit of not being stuck without a vehicle, and got $8K more by selling it myself (vs the trade in offer).
Hopefully this helps some of you if you're contemplating a similar plan.