Welcome Mike
there are really only 2 basic ways to connect cabinets, most original cabinets on
fiberglass rv's are installed using rivets or other fastener through the
fiberglass shell from the outside, this is fast, easy and effective but any time you drill a hole through the shell there is the possibility of a leak, this can also cause a more concentrated stress at each fastener which can start to crack, although this cracking is usually just in the gelcoat.
The second method and the one I refer is to epoxy blocking or anchor points to the inside of the body and connect the cabinets to these, very clean look, no holes and not
leaks, it also spreads the load over a larger area and reduces the potential for stress cracks