I never heard of mouthwash before? You have gotta wonder who is the first person to try these exotic treatments,,,"like maybe I will soak him in mouthwash...yeah, let me try that"...
I would try and find either Provent- A-Mite, or I have heard good things about NIX from Wal-Mart and it is like %5 the cost of P.O.M.
Read the directions for PAM if you go with this product. I havent had this problem in 3-4 years, but I would remove the animal from the enclosure and spraye enclosure down good. You can take Provent and spray it on a clean cloth and gently wipe the animal down as well. It is VERY important to air out the enclosure very well before re-introducing the snake.
Keep in mind that this initial treatment will kill all the living mites, but not the eggs. They will hatch out over the next week or two and you want to retreat at some point.
You should go to V.P.I., the Barkers web-site and then look under publications. They have an article about The life history of snake mites that is a must read.
Good luck.
p.s. Reptile Relief sucks also, dont even waste your money on that crap.