I'm all for solar, don't let me talk you out of it.
But why upgrade to 4x6V batteries? That is a lot of capacity, what is it needed for? If your camper power needs are "ordinary", as in lighting, water pump, furnace, and fridge, then 2x6V could last you through a long weekend without any recharging if you're being frugal with it. The 200-300 watts of solar that you mention is a good match for 2x6V batteries and would eliminate any worry about running out of 12V power if the sun is shining during a long weekend camp trip. So why the interest in doubling the battery capacity? That would mean you would need to double your solar wattage to 400-600 watts to make it useful for recharging that much battery in a reasonable amount of time.
I have not heard of a waterproof controller, either. I mounted mine in the pass through storage located at the front of my camper. I drilled a hole through the floor to pass the wiring through for a short wire length to the batteries on the tongue.