The “port” that the OP described is the only type I’ve ever had. As mentioned, air vents out the hole next to the port. I just shove a water hose in the port and fill to the desired level. The hose stays in place by itself on mine. If I want the tank full, I fill until water shots out, then put the cap on after the pressure has relieved itself. I’ve never had a problem with it.
2020 Sunseeker 2440DS Ford, Trekker cap, Topaz paint