<snip> but I figure in the event I am using the camper offroad, any severe sloshing around would simply leave the camper through the vent to the outside. Plus I really don't want to see the vent inside the camper and in order for it to function in my rig, it would have to be mounted above the cabinet/seat that houses the tank. <snip>
I believe my vent tube is 3/8" ID. That is a very small volume, therefore water from a large container can move quite a distance up it. When my tank is filled, I am always seeing water coming out the outside vent - visible in the driver's side rear view. This is especially true when driving mountain passes and on any kind of rougher backcountry road. I would not vent inside the camper.