Venting a water tank

[Loud Al]

I'm building a trailer and I'm going to run a 25 gal tank with an electric pump. The thing I'm trying to figure out is how to vent the tank in a way that I don't have to worry about losing my water on steep uphills or down hills. I'm thinking a check valve but this will only work for draining the tank, when I fill the tank the water will displace the air and that air needs to go somewhere. I did a bunch of searching but I can't find anyone that mentions this issue.

Thanks

 

[Javelinadave]

Easy, a ball valve. Close when driving and open when you want water.

 

[Titanpat57]

RV waterfill/vent combo....obviously mounted higher than the tank.