I think that the best way would be to make sure your water and grey tanks are empty. Then fill the water tank, run your water pump until the water tank empties into the grey tank. Then dump the grey tank into a 5 gallon bucket repeatedly until empty. You'll learn two things, the capacity of the water tank and if the grey tank will hold the capacity of the water tank.
You could also time how long it takes your hose to fill a 5 gallon bucket, then time how long it takes to fill the water tank.
Caution, don't do this in California due to drought restrictions. You could also put bleach in the tank and sanitize the system while measuring capacities. Also, some RVs plumb the lavatory sink into the black tank to add liquid to the black tank and slow filling of the grey tank. Start with both grey and black tanks empty.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI