You will get more info on your chassis my asking on the Workhorse Owners forum here at IRV2..
The answer is, In front of the right front tire. You do not need to jack it up, just turn the wheel to the right and you can get to the filter. I do not think anyone has used conventional carburetors in year.
2004 National Sea Breeze LX8375, Towing a 2012 Liberty.