Can I use the same electrical ground for everything in a van?
After doing lots of research, I came up with the answers myself.
Yes, I can and should use the same ground for everything. The current will automatically flow only where it's needed. In my case it's important to use a good relays which limits the amount of electricity flowing between the batteries so that they are not short circuited.
It is also recommended to use the chassis as the ground because I'm using fewer wires therefore reducing the weight of my vehicle.
Debugging solar panels can be done by connecting both sides to an ampere meter (multimeter). This won't damage them as long as it's done one by one (and maybe even if it's done in series). The voltage will be normal even in broken panels, but the current will be very low. So this way I can eliminate the broken panel(s). If both of them work, the fault lies at the charge controller.
Sources: https://www.youtube.com/watch?v=IPq_d4ftYnM, Solar panel short circuit, Using the frame of an RV trailer for the ground, Why is the chassis used as ground in automotive electrical circuits