Not the most scientific method but Ford equipped the 20,500 lbs chassis with 19.5" tires while the 22,000 lbs chassis has 22.5". Also the rear axle is different between the two but a bit more difficult to determine. The 20,500 chassis has a Dana 135 rated at 13,500 lbs while the 22,000 chassis has a Dana 145 rated at 14,500 lbs. I thought a VIN decode would state the limit but I ran a check for my coach and no luck concerning the chassis GVWR.
The stick with the Ford logo for the incomplete chassis as delivered is the one I would go by rather than the couch builder.