If the park has wired Internet and your trailer doesn't, all you really need is a travel router. We haven't needed it, but for hotel travel I've used an Apple Airport Express that you just plug into a wall, then plug into an Ethernet port. Once it's plugged in, it works like your own, private WiFi LAN.
For trailer park use, all you'd have to change is, instead of plugging it into an outlet and Ethernet port on the hotel wall, you use the plugs on the power post, where it'll sit hidden by the rain cover.
Five Best Travel Routers