Recommendations on best water flow meter

Heading to the high country in the morning and want to measure how much water actually goes into my holding tank before I leave in order to control weight load and economize on fuel I burn to get there. Knowing that each gallon weighs 8 lbs there must be an accurate meter out there that will do this for me. It seems the more popular digital meter sold on the internet is said to be off by 11% which would be no better than just guessing how much is going in there. The water tank on my old travel trailer was under the sofa so all I had to do was look under the sofa and do a visual. My 5th wheel has the tank located below which is sealed out of view and the little light panels on the wall only tells you when you are at 1/4, 1/2, 3/4 or full levels. I question their accuracy. Any ideas on a good meter so to better measure the water that goes into my tank without having to do it manually?