I would expect Royal to have ordered standard 40' fuel (160) if they were going to add their own tank. If their aux. tank is 60 gallons that would give a 40' Royal 220 gallons. It would be rare for a coach to have a Prevost aux. tank, and a second converter aux. tank. We sold a 45' XLII Thompson with standard Prevost fuel, that the owner had rigged up a removable extra tank that could be positioned in a storage bay. It was not installed when we sold the coach, but I believe it added 75 gallons or so of capacity. It was similar to what you see added to aircraft that are being ferried overseas.
Steve & Rhonda Bennett
Dana Point, CA
92 Marathon XL40
2012 Jeep Rubicon Unlimited
Owner: California Coach Company, LLC