You can have the tank on the trailer, then you will need a 2 line system, BUT not the same 2 lines as a floor, they work in different pressure ranges.
The hydraulic comes off of the Power take off on the transmission, it can be a direct drive (most popular) or a shaft drive, no real difference, just some guys prefer the shaft drive, as for ease of mounting mounting.
Trailer mounted tanks are not as popular because then you would be limited to using the same truck all the time, plus the pump works harder sucking the oil from the trailer tank into the truck pump, where when the tank is close to the pump it is easier on the pump, that is why they recommend mounting the tank on the deck plate behind the cab, so the oil can gravity towards the pump.
I have never seen a walking floor trailer with a tank on it, but that doesn't mean it can't be done, but it also means having to buy 2 tanks.
Look here
Chelsea Power Take Off PTO Units & Parts..
And here
Muncie Power Products: Power Take-Offs, Fluid Power Products, Hydraulic Components