There are two little rubber plugs on the same side as the USB cable port with TS9 antenna ports in them.
Depending upon what kind of antenna you get, there are various adapters for those ports.
At the very bottom end, something like these will give you a small amount of increased signal, but will help to get the signal above the RV, and outside of the walls (with these adapters).
This one is the simplest, ~~will plug right in~~ - https://www.amazon.com/gp/product/B003NPYON8/
This one will require an extension cable and adapter, but is more powerful - https://www.amazon.com/gp/product/B01LVTJR7Z/
This one is the same, but directional (you will need to point it to a tower closest to you) - https://www.amazon.com/gp/product/B00J14YEHQ
Extension cable - https://www.amazon.com/gp/product/B00QZEQ5EA
Adapter - https://www.amazon.com/gp/product/B00HG7HBDY/
I use all three of these currently, but mainly the Wilson Electronics 4G Omni Plus Building Antenna.
Remember, your Nighthawk has two antenna ports. You won't gain a benefit from having two, but you can always have two plugged in. This way, the one with more signal will be in use (at least this is how I understand it to work).
Edit - sorry, you will need the adapter for any of these. The first one will not need an extension cable.
I'm using two of the WeBoost / Wilson Mini Mag Mount antennas (the picture is of the older 3g model, but the regular 4g version is what was shipped).
https://www.amazon.com/gp/product/B003NPYON8/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1
And these adapters:
https://www.amazon.com/gp/product/B00PP0MEBS/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1
I'm using them with an AC791L on Verizon, so if you have a different device or carrier, you may need something different.