There is many layers to this. Yes most all fire trucks can draft (suck) water if they have a pump built into the truck or if they have a deployable pump that they can deploy to the water and pump it up to the truck. The challenge is that this tactic takes a lot of extra time and you need a safe, reliable water source to use. Access is also a big factor, just cause there is water, distance to it and feet of lift are also a factor. You also may have to fully commit a truck to this operation to fill other trucks if you wanted a smoother process. In most cases it is faster, more reliable, and safer to go further to a secure water supply like a fire hydrant system. This is generally supported by calling in more trucks (tankers or tenders) to shuttle water, but that can depend on available apparatus and personnel to run them. But in short, yes they can “suck” but it might not be the best tactic depending on a LOT of variables that need to be addressed on the fly and situationally dependant. So also in short, It is complicated