mjrleague,
Looks to be 25-30'. Do you have to polish the diamond plate? Any chassis-blacking? Inside?
How many hours do you think you will need to complete the job? Charging by the foot is commonplace for RV's, boats, big rigs... May not be a bad idea to approach the owner that way.
Have you seen the condition of the roof? If it's not caked with bird mess, tar, black streaks... then you should be able to wash/prep/wax the same way as the sides. Going at it with a good soft bristled nylon pole brush during the wash phase is a good idea.
You should not bother with the clay as this is most beneficial on modern day auto paints (rather than fiberglass or gelcoat) to remove embedded particles.
Get some rest,
Mike
Visit your local boat or RV supply store for gelcoat, vinyl.... many carry great products by major auto wax companies that are formulated for this type of job.