Hi. Being you have a US passport is supposed to entitle you to a same day visa in BKK. Last year I tried to get a same day visa in Bkk but since I hold a Maltese passport they told me it will take 3 days not like Americans who get it in one day. After giving me this visa, I reapplied
a couple of months later for another visa and they issued it same day. If I remember correctly you got to go to wireless? road very close to the American embassy. Price if I remember correctly was 2500baht and dont forget your photo. Good luck
PS . Not much to do in their airport