In the absense of specs for your rv best method to confirm tank capacity is to connect a Save a Drop P3 turbine totalizing
water meter https://www.amazon.com/Save-a-Drop-P...60933090&psc=1
to a water hose, fill the tank and record the total gallons.
If you can access your tank measure the height (The bottom of your waste tanks slope towards the outlet. Measure the at both ends and use the average of both measurements for the height). The multiply average height x width x depth in inches will to get cubic inch volume of the tank. Divide by 231 to convert to gallons.