It's my understanding that the "factory Ford option" for CNG/Propane is composed of different valves/valve seats and a "dual fuel intake manifold" (ie: engine changes) and that the actual CNG/Propane tank, fuel lines, regulator, etc are NOT included. Cost is listed on the Ford website as $315.
If the vehicle you're looking at has been "converted by the dealer" to actually operate on CNG/Propane, then he has added the "missing components" at a "substantial cost". Don't forget that there are very few places to refuel with CNG and propane is not yet "taxed for highway use" in many areas (making it difficult to find for "legal use").
Don't forget that with an F250, payload is a factor for potential fifth wheel use and any added weight from the CNG/Propane system will come at a cost in vehicle potential use as a tow vehicle as the payload will be substantially less than a "flex fuel" (unleaded/E-85) fueled vehicle.
Also, I believe the towing limitations are significantly reduced on the CNG/Propane fueled vehicles when they are being operated on CNG/Propane, so you may not be able to actually tow when using that fuel system.
I'd do a heck of a lot of research before buying an alternative fuel vehicle as a primary tow vehicle !!!!!
ADDED" Here's an article you might find interesting:
http://news.pickuptrucks.com/2012/06...d-bi-fuel.html