fresh water vent

[Pete C]

Hello all, hope everyone is doing great!!
I have a 2017 Keystone Alpine 3900re, on occasion I need to top off my water if I'm not going to be at a RV resort. I'll fill it up until it comes out the overflow vent which shows three dots on the inside panel. by the time I get to where I'm going I have only one dot left in water capacity as it spills out on turns and so forth.
My question is can I put a valve on the vent line while traveling and then open it when I get to my destination or another suggestion?

Thanks Pete

[JRTJH]

Rather than trying to plumb in a valve, I know a lot of people who just put a rubber pencil eraser in the "little vent hole". Now, if the water is sloshing out of the actual fill opening, maybe a rubber bathroom sink stopper ??? I'd look at "plugging the hole" rather than "altering the plumbing"... Much easier fix, IMO.

[fjr vfr]

Seems strange to me. I've never had water spill out the vent tube on either of my trailers.
Are you sure you aren't trapping air in the system and having it burp out as it moves?
Also causing a false full reading in the beginning?
I usually have water burp out during filling and have to let it burp before continuing with filling.