I put the extension I already had on the hitch and it looks like extending the shank by 10 to 13 inches will allow the tailgate to come down. However, I don't want to use an extension so, I  talked to Airstream and a reputable trailer and hitch dealer and will try a longer shank ball mount. 10 to 13 inches of additional length will work. I will get the shortest length I can. If I rotate the electric jack 90 degrees I can shorten the new shank even more. I don't know what lengths the shanks come in but I may be able to get it down to only 10 inches or so.
Here is the 12 inch extension which I will replace with a longer shank ball ...   
