I also have 2, 6v batteries. Our panels are 200w total. They keep the batteries topped off really well. Larger panels would be heavier. Ours weigh less than 50#. Check Zamp, Renogy and Goal 0 for some quality manufacturers worth looking at. We got lucky and found our Zamp kit used and saved over 1/2. Folks I've talked with find 100w panels working well for them.
Not sure what plug is on your Passport but I'm thinking it's probably a standard SAE automotive plug
https://www.amazon.com/iGreely-Autom...C5FTBKAEQX55P4
Zamp uses this type plug from their controller. Obviously you could splice this plug type onto any controller's output wiring. be sure to use 10 gauge wiring from the controller to the connection to minimize voltage drop.