You should not have to add any grounding at the distribution panel. The distribution panel has only 1 DC ground that all the circuits pass through. As far as adding the circuits are concerned, this is what I would do to determine if you can add them:
1. Throw on all your DC load.
2. Use an amp meter to determine how much current is passing through the ground wire with all your DC loads on.
3. Run the new circuits you want to run on the floor and plug in the devices you want to be able to charge. As far as grounding is concerned, try tapping into nearby DC grounds. Make sure that the ratings of ground wires you're tapping into can handle the additional amperage (if you're able to add the circuits permanently, I would break them up among your remaining circuits in the distribution panel. Laptops can use as much as 10A to charge, so you may run into a problem with that. However, iPads and smartphones only use a couple of amps).
4. Remeasure the current in the ground wire in the distribution panel. If you have room to spare within the rating of the ground wire in the distribution panel, you can add the circuits permanently. If you don't, do NOT add the circuits permanently.
5. Consult RVIA for the proper gauging of wires that correspond with different max amperages (i.e. 15A = 14Ga, 20A = 12Ga, 30A = 12Ga).
The distribution panel I believe has a max amperage of 30A for the first 2 circuits and 20A for the remaining circuits, but consult the manual on the distribution panel for verification.
If following the above steps yields that you cannot add the circuits and your unit has
12V accessory sockets, I would use those. If your unit does not have those, I would use a wall adapter on an inverter circuit while dry camping.