Looks like a very well designed frame.
Are you always going to be carring 500 Gallons of water ( 4,175 lbs ) plus your 2 washers if you put on the burners ( 1,700 lbs) 4 hose reels ( with hoses ( 400 lbs aprox) 2 surface cleaners ( 100 lbs) Plus odds and ends. Thats approaching 6,300 lbs and water weight is considered a different animal than dry weight. That would be murder on your braking system alone.
I am sure that you figured all the ratio's out though.
For those who have not figured it out with a cat program, you need to consult with an engineer when you are approaching the 85% or more for the pay load limits for safety reasons. Also check local and state / FEDERAL DOT requirements.
Great setup. Can't wait to see it.