Are plastic 5 gallon gas tanks suitable for use in vehicles as the main tank?

If you can pump gas at the right pressure to the engine it will run.

Extremely dangerous.

The only thing you should use as a fuel tank is a properly designed (and installed) fuel tank or fuel cell.

You can't legally do that. Forget it.

It's been a long time since you could buy the portable red gas cans with the little yellow vent on them. Modern gas cans have no separate vent - they "vent" through the spout by design when the gas is being poured out. So no, you wouldn't be able to use a portable can as a primary container and pump fuel out of it without modifying it. And do not modify a portable can! Just spend the money and buy a tank that was designed specifically to have fuel pumped out of it as a primary tank instead.

No, that would be dangerous when carrying it around in the car, truck or whatever as fumes and indeed fuel itself can escape into the car. Those intended as outboard fuel tanks, though, do have an air admittance which you'd have to screw open before use in the boat.