mfisher, welcome to iRV2.
The fuel filter on a Workhorse chassis is located on the inside of the frame rail near the driver's side rear wheel.
Your filter will probably have one line at the rear and two lines at the front. These filters are quite expensive and are not available at local auto parts stores. A good source is from one of our members oemtech
. Scroll about halfway down the page to Workhorse fuel filter. There is also a reference on that page to an adapter that will let you use a less expensive aftermarket filter. Use of the adapter is not officially approved by Workhorse, so the choice to use it is up to you.
There is no suggested mileage to change the filter. One bad tank of gas could completely clog a brand new filter. Since the Workhorse filter is not available from the auto parts store, and since a Workhorse dealer may not have one in stock, it is a good idea to carry 1 or 2 spares.