The "world according to Gus" (again). To keep with "code" you must ground your WFCO converter to as much of the metal on your trailer as practical. This is code. If you verify that your SS bolt to frame is electrically conductive with a meter it will be fine. However, ground connections are notorious for failing and should be checked and "serviced" on a regular basis to ensure that the connection is good.
Don't quote me on this paragraph but I believe the ground wire must be bare solid copper wire. Not sure on the gauge it should be but if you are plugged into a 50 amp service you couldn't be underkill with #8 wire. Perhaps someone more familiar with the code could comment on my comments that I'm not quite sure about.
Running your trailer off your vehicle's batter is, IMHO, not a good plan. Your vehicles starting battery is designed and manufactured for starting your car. It is not meant to run a continuous load for anything but a short period of time. If you are boondocking, run your vehicle battery down, and no one is around to give you a jump, you're gonna be SOL.
A battery is maybe 60 pounds. I suggest you carry 60 pounds of stuff in your tow vehicle, that you were planning on carrying in your trailer, if you're that concerned about your trailer weight. Just be careful not to exceed the total weight capacity of your vehicle and trailer.
If you're gonna be toodling around during the day, you can move your trailer battery into your tow vehicle, with an auxiliary connection of some sort, and charge your trailer's battery as you toodle around.
Cheers,
Gus