Jon got it right --- probably.
OK some times the manufacturers will have other guidance and if they do follow it. But one thing to understand is that ground is just an artificial concept. It is just a reference point.
If you get into doing drawing electrinics out there are actually two types of "ground": "earth" and "chassis" and both can be other things than you would think. As an example I have done work where if you were to find a way to safely measure the difference between a stake driven in the ground and the "earth ground" of the equipment we were working with you would actually measure several thousand volts. Yeah special science stuff, but the point is that as long as you have the reference you are good. Chassis ground is basically the ground within the box of electronics you are working with. A lot of the time the chassis ground is connected/bonded to the earth ground which yes usually is the earth but now always. I have done work where if you were to put a ground rod in the ground 500 foot apart and then put a meter between them it would read several volts.
So one thing is that you want a single ground to be your ground. If you were to take some equipment and hook it up with wires between it and then hook it to earth ground in both of the locations mentioned above it likely would fail because there would be two grounds.
But as I said ground is just a point of reference. I your case you it does not sound like you need a connection outside of the box. As long as all of the things that need to be connected to "ground" are all connected to a central point with big enough wires that there is minimal resistance then you will be golden.
Then the comment that Jon made about the camper frame. Now in a way your camper is a box of electronics just like say a transistor radio. If you were to have a old transistor radio it would have a case and then everything in that case that needed to be connected to ground would be connected to the same point. And that is great and makes it work just like any other piece of electronics you have. Now if you connect two boxes then their grounds will most likely need to be connect to each other. Add another box and its ground also would need to be connected to those grounds.
That is the principle of the camper trailer. There is nothing magical about the camper frame. It is just a convenient point of reference therefore a point of connection for the grounds of all of the various boxes in your camper. You could in fact have nothing in the camper connected to the frame and still have everything work great as long as they were all connected well to each other with big enough wires and good solid connections. But the secret is that the trailer frame is like a giant huge diameter connector which runs everywhere along the bottom of the trailer. So if makes a really convenient way to connect all of the grounds and have them connected with big enough "wire" that there is very little resistance in getting from front to back. So the "ground" is the same all the way through.
So if you pick your box up and put it in the camper no problem as long as it is well connected inside no other ground would be needed. Lay it out on the ground out away from the trailer no problem no more ground connection is needed. If you hook something to it that is not connected to anything else to power it, and you connect the ground of that device to your case's ground you have what you need. But if you connect it into something in the trailer that is connected to the rest of the
electrical things in the trailer then you need to make sure you solar box's ground is connected to the trailer's ground, most likely the frame directly or indirectly.
Now about these connections. They need to be as big or bigger wires than what is needed for the rest of the wiring in the system. If your power like is coming out of the box to power things is say 10 gauge, then you will need the wire that connects it to the ground of the items being powered with at least a 10 gauge wire. In many cases good design is to go to the next size bigger such as say an 8 gauge wire.
Then to the actual connections. Remember you are dealing with an RV. something that moves. So the wires and the connections have to support that fact. Frankly mostly I solder the connections for things that I have that move as much as possible. If you have a bolt or screw making the connection, make sure that it can't come loose with vibration or abuse. And generally stranded wire is more compatible with any item that moves.
I know this is a lot to read, but hopefully this will help you understand what you are dealing with and how to deal with the question and those people with their answers to this question.
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