I chuckle when I see "550 gallons".. it seems a ton of city departments go with the smallest tank they can fit into the chassis simply because they have hydrants in 90% of their service area. Doesn't do much good when you are in a park or a rural part of the city. Interestingly enough it looks like they managed to find 50 more gallons of space somewhere, compared to the older trucks.
My department's new engine, plus the one recently sold off, each held 1000 gallons. The other engine we have, which is a retrofitted/refurbished city engine, carries 710 gallons, only because that is the absolute maximum we could squeeze into the chassis (and that is with a custom-designed tank to fit around the components of the body and Squrt tower).
I also found the seating for 5 interesting. We spec'ed our truck with 6 seats because it's built to have an even number of seats in the rear. Whatever floats your boat, I guess.