find a better expert to help you.
start with local Victron dealers, if no luck find the local Victron rep and ask for a good dealer recommendation.
GFCI do not need a ground to work, so anyone that tells you otherwise don’t use them.
all battery negatives should be connected (unless there is a very specific case for an isolated system)
this is beyond a quick online help thread, someone needs to get hands on that knows what they are doing to help you get this properly sorted.