OP said "P-brake button". That would indicate Workhorse P32 chassis with DRUM BRAKE on the end of the transmission.
So the hydraulic release pressure is not over coming the brake apply spring.
To the OP. Chock a tire, then go under your coach and remove the cable pin, near the drum, that applies the E brake. That will allow you to drive to a service center. You will NOT have park or an E brake, so have you co-pilot chock the tires when you stop.
Richard
95 Bounder 32H F53 460
2013 CRV Toad
2 Segways in Toad
First brake job
1941 Hudson