My money is on the full Banks Power Pak System. Are you on the Ford or Workhorse Chassis? I see the '06 Admiral came on both chassis. The Ford will benefit more from the Banks, but there is quite some power improvement with the Workhorse as well.
Banks IS expensive, but I believe worth the investment. Since you are in wet, slushy environments quite a bit, the all stainless steel Banks will last longer the the RV!
Should run around $3400, or so for PowerPak w/Trans Command.