I am learning a lot from this thread. I am learning about things I never thought I wanted or needed to know about...watch out Jeopardy :-)
Terry I hope you are able to get this figured out easily with much additional expense and/or labor.
After reading this I did a google search on Votage Drop and read several articles. Then I searched for a calculator, the first hit was from "South Wire". When I input your data from the original post, it determined #8 wire if direct buried was required to maintain <3% VD. They calculate actual VD to be ~2.32%
2007 Chevy Duramax LMM/Allison
2012 Jay Flight 29QBH
2012 Jay Flight 26BH (Sold)