So just an idea I had while looking at apparatus images.
What if like the two types of engines, there were two types of ambulances? They would both do the exact same thing, but this one could carry say 4 personnel. Here's an image for reference:
[Blocked Image: https://photos.smugmug.com/NorthCarolina/MecklenburgCounty/MEDIC-Mecklenburg-EMS-Agency/i-Cn67xDQ/0/eba9eaab/M/MecklenburgMED30-M.jpg]