Go to the NAPA web site: https://www.napaonline.com/default.a...FQgqIgod9EfN6g
Please note that the new NAPA web site is super slow. Far to many advertizing graphics.
1. At top of screen, enter your zipcode in the My store box and select one of the stores as Your store
2. Add a vehicle - NAPA has the motorhome chassis mixed in with the P30 Step-Van category. So select year 88 (if that is the year of your chassis), Chevy Truck, P30 1 ton Step-van.
3. On left side of screen select Brake Systems Filter.
4. GM produced both 4 wheel disc and disc front/drum rear versions. Select the appropriate filter on left side.
This info is from a 84 manual but should still be close:
The JB8 chassis is Hydroboost with front disk (12.5" rotor) and rear drum (13" x 3.5")
The JF9 chassis is Hydroboost with front disc (14.25" rotor) and rear disc (13.75" rotor)
That will get you in the ball park. I do however recommend when you go to the parts counter you confirm everything (most likely in a paper parts book) then visually double check prior to install.
You do know that rotors can be turned (reconditioned) if not worn down to far?