Maximize Reception of EX7000

Hello, I just got the EX7000 to pair with a Nighthawk R6700 router in an RV. I am using the EX7000 to pick up the local campground signal, and let the router capture the campground IP address. This way I simply log into the EX7000 at the new campground and add the campground WiFi info and then all of the devices are good to go from the router info that doesn't change.

So this is working decently but I would like to try to maximize the reception of the extender. I tried to use an existing external antenna in the extender, even trying it in each of the 3 antenna ports with no discernible difference in signal strength. Has anyone had any success adding a more powerful or external antenna to their extender? The existing external antenna is the KING Swift Omni-Directional Wi-Fi Antenna, installed to the roof at the factory, but I had so many issues with the original system it may be that the antenna is trash anyway.

Thanks!