With new A/C components and a fresh recharge I had 40* at the vents, Max A/C on, 90*+ day, 2000 E250 5.4 motor.
These days in my '03 E250 extended body with separation curtain I can still get 43* or so with original A/C system, topped off and reading correct pressures, normal A/C selected. Both systems with R134a
Your charge could be low but without reading the pressures its tough to tell if that's the only issue. Since you don't mention the year or mileage of your F250 this is just a guess.