NJ's EMS task force has a large mass care vehicle and small mass care vehicle:
https://www.nj.gov/health/ems/documents/special_services/njemstf_overview.pdf
Wake EMS has Major Operations Support vehicle Truck 1, as well as their medical ambulance bus EVAC-1
Most MCI vehicles are better suited as county wide resources, vs individual agency vehicles, simply because most small agencies can't justify an MCI truck just for them.
I've seen two types of MCI vehicles:
trailers that are pulled by a pickup truck, and
MCI vehicles. The biggest advantage with vehicles is you have power and can charge equipment (and you don't need another vehicle to pull it), but they are substantially more expensive.
If I were specing out a MCI truck, I would make a list what I would need to handle the following incidents:
1) highrise fire with 20 patients with smoke inhalation
2) fully loaded bus vs another another bus traffic accident
3) music festival / school sporting event with either OD or weather related MCI
Things to make sure you have:
collapsible cots
chairs & tables
fire rehab equipment
shelter from the rain
extra water and gatoraid
Triage supplies (red/yellow/green signs and colored tape to identify boundries)
traffic cones
FEMA paperwork and container to store it all
back boards (to move people from point A to B, not to strap them down and torture them)
The real answer is, you can spend a whole lot of money on something that might get used maybe a dozen times a year. Look at your area, and determine your needs.