Ford put two fuel tanks into their E-350 and E-450 chassis.
Winnebago might have selected the smaller tank in order to have more cargo capacity before reaching the GVWR.
I would suggest measure the tank, and then find out how many cubic feet it is, then what gallon capacity that works out to. The 40 gallon tank will have outside dimensions that indicate it can hold 40 gallons of fuel and 8 gallons of vapor, or 48 total.
The 55 gallon tank should hold about 55 gallons + 11 gallons of vapor, or about 66 total. It still might measure out as about 70 gallons.
Fred.