I've driven several tandem water trucks at 80 bbl (3360 gallon) and the chassis depends on the service.
What year range you looking for?
Typically I would recommend a Pete with Pete air in the rear as they offer the best ride but it suffers with off road handling as you will spin the rear axle easy and get stuck. With careful driving in the mudd it's fine and this gives you the best all around truck.
If you drive a lot of road or in mudd and snow conditions a typical walking beam truck is excellent but you suffer unloaded on the ride.
I've driven a few macks and if this was 1998 I'd tell you to get one of those, but anymore with expensive replacement parts and the newer trucks suck I'd stay away. The last truck I drove was a 1998 CL with a e7 460, nice old truck that road well with camel back. Macks can have 3 ptos for a Bowie pump, vac pump, and a high pressure pump.
As far as manufacturers go you just need to go with whatever severe duty truck you want to go with. My friend bought a new Western star with a dd15 for his new truck, went all hydraulic for the pumps which is the way to go anymore.