Welcome to the forum and to Keystone RVing. From the question you posed, it's easy to see that you don't have much RV experience. With time, that will change. It's been said a number of times, that the only time RV holding tank gauges are correct is when they come from the factory, have nothing in them and the gauge reads empty. After they are filled the first time, plan on them being wrong for the rest of their lifespan. The reason for that is that the gauge sending units are simply contact pins that sense the liquid between the contacts to signal how full the tank is. Once you fill the black tank the first time, small bits of solid material and tissue adhere to the contact points and they send "false signals" for the "rest of time" Well, they send false signals until we stop using the system, then they just 'sit there".....
Do a forum search using the top bar search feature and type in sending unit, black tank gauge, tank gauge or a similar term and you'll get lots of reading material to urge you not to fight the tank gauges, it's a "never ending battle" LOL
Each of your gray tanks should be 40 gallons. They are usually identical and one drains the kitchen, the other drains the shower, lavatory.
Welcome again, enjoy your time on the forum and we look forward to hearing from you often.
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John
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS
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