I saw a guy with a setup like this once, he had 2 ramps that he hung on the back of the 12 foot flatbed, and a ramp in the bed up to cab height with a small electric winch to pull the boat up the ramps, was a pretty sweet setup until he tried to drive up the Westwater launch ramp empty. It was only a 2WD truck, and he helplessly spun the rear duals until I hooked a strap to him and drug him up the ramp. I confess I wondered about what the boat sticking up over the cab did to the already poor fuel mileage too.
Flatbeds typically large enough to have a 12 foot bed typically are built on 1 ton chassis trucks, so stiff suspensions, poor fuel mileage on almost all the new ones, be it gas or diesel, and cost a fortune to insure due to the damage all that weigh moving at speed can inflict, not to mention 30 to 35K on a set of 6 tires, brakes at 40K, they are expensive to own, I've had a few of them over the years.
You might do a cost / benefit analysis, seems that no matter how I look at it, a trailer seems the cheaper and more versatile option, your problem being where to store it.
There are a lot of boaters in the PHX / FLG area, perhaps sign up to the Central AZ Paddlers club yahoo group [email protected]
and making a post seeing if another boater has room to store your rig in a convenient spot for minimal cash outlay?