Steel v/s Aluminum v/s Stainless v/s Poly for Fire Truck Construction

Over the past few months I have been investigating replacing one of our trucks with a new unit. For a little background the department is looking at replacing a midi-pumper with a flatbed or mini-pumper type truck. The question that keeps popping up is the material for the fire body. We have always had steel body trucks but there are other options that include aluminum, stainless steel, and poly material. Just wondering about what works out there. I know some of the advantages and disadvantages of most of these materials but do you have more. Any input would be appreciated.

Steel-advantage is strength, lower cost, ease of fabrication
Disadvantages are corrosion, weight

Stainless Steel- Advantages are strength, corrosion resistant,
Disadvantages are weight, high cost, slightly harder to fabricate

Aluminum-Advantages are corrosion resistant, good strength to weight ratio, lightweight, ease of fabrication
Disadvantages are may be prone to stress failure (I wonder what the statistics are on this and if it really is an issue in fire bodies?), have heard some departments with older aluminum trucks complain about paint issues with the aluminum, cost falls between steel and stainless.

Poly-Advantages are lightweight construction, ease of construction, no corrosion problems, impact resistant to resist damage caused by minor fender benders etc.
Disadvantages are stigma related to having a “plastic” fire truck, lack of long term service record to show how well they hold up OVER LETS SAY A 20 YEAR LIFE SPAN.

(I really don’t have enough information or knowledge about poly to really discuss it intelligently so I listed things I have heard, some of these are “around the firehouse” conversations so take them as that.)

I know that some manufacturers specialize in fabricating fire bodies from different materials and some offer various materials as alternates but I really don’t want to get in a “this manufacturer is better” contest. I would just like to see what’s working and what’s not working with your fire truck construction. Our problem with the steel construction is corrosion and that’s not that big of a problem. We try to keep the compartments dry and keep water from pooling up anywhere in or on the body and that helps. The biggest advantage I see with aluminum is the weight savings and the corrosion resistance. As I stated above our hope when buying a truck is a 15-20 year service life with maintenance and equipment updates being performed as needed, so longevity is very important. We have had a very good record in doing this with the past trucks