If you have access to compressed air, as others mentioned, try to blow air into the vent tube and also into the drain tube. If you notice an immediate improvement, that means you blew out an obstruction. If it returns, it means the obstruction found it's way back. It could even be a piece of plastic that never completely broke away when the hole was drilled. It could be hanging inside the tank right at the hole. It could be flopping around, immediately returning to obstruct the hole.
Such little irritations do occur with most motor homes, Phoenix Cruisers and all the other brands. Once we get past them, we generally do fine. Sometimes we will look at what went wrong and say......."Come on guys. It's not hard to do this right." That's the price for non-automation, "hand crafted" products. Still, a lot comes down to engineering & design, manufacturing processes, training, inspecting, and every worker taking great pride in their work. Phoenix USA does much better than most, but obviously not so good when it comes to the fresh water tank that many here have similar stories with it.