Here's an easy way:
1st step:.You can always "time" the input of the hosepipe to say, 30 seconds. Pull the drain and measure the runout amount. Armed with this, just fill the tank (watching the elapsed time interval). Divide to get the capacity.
2nd step: drain off exactly 5 gallons & leave the remainder in the tank.
3rd. step: add **diluted bleach to form 5 gallons & pour this into spout
4th: Move rv so that it splashes on roof of tank. Let it set for hours.
5th: rinse with fresh water, [vinegar or baking soda ..if necessary for taste]
**[Assume you know "not" to add full-strength bleach; rather dilute it extensively before it enters the RV system.]
Here's an excellent primer on mix ratios, courtesy Tiffin RV.Network