We have some "interesting" scratch-built fire fighting apparatus around here in some of the smaller communities of Northern California.
But one of the largest water tenders of any Fire Department in this area, would be Mountain Gate FD's WT-41, which is a 7,000 gallon water truck on a Sterling chassis. (Post #26).
A private company that contarcts to the USFS or CDF during fire season owns two monster water trucks out of the town of Burney, a community in the eastern Sierras. One of them is an Oshkosh with a ridiculously large water tank and the other one is a scratch-built hybrid water truck built on a military 5 ton 6X6 chassis. It has a Detroit 318 (8V71) shoehorned into it and it's aptly nicknamed "The Beast from Burney".
It's also not uncommon to see contractors out on the line with water pulls like the Cat 6xx series 10,000 gallon (average) articulated water movers.