Divide HP damaging parts from armor parts. Use vehicles external geometry as armor and divide fire geometry into the internal parts only.
Apply armor RVmat to external geometry with appropriate values all around. By leaving openings or applying particularily flimsy materials, particular weak spots can be simulated. (Shot traps around the turret, very weak top and bottom armor, lightly armored hatches, windows, etc.)
Model the fire geometry where actual damage will be dealt according to distribution of internal parts on the actual vehicle. Assign degrees of vulnerability. What I imagine to happen is this:
Shot hits the tank, penetrates the externals through the RVmat RHA, and carries the damage into one of the internal hitboxes if it strikes one of them. Any strike should do some sort of damage, if only wounding the crew.
Armor should not degrade. Hull damage should simulate condition of internal hitboxes and not external state of the vehicle.