Well, working under the 3" steel beams to keep the 1/4" per foot slope was a challenge but it's done! The P-trap for the shower had to hang lower than the galvanized sheeting under the trailer so I boxed it in and insulated it. Looks super clean and is accessible for service if need be.
Instead of trying to stuff insulation under the ABS pipe and lose R value, we foamed, which cradles the pipe, keeps the slope consistent, and protects the pipe around the pan penetrations. (plus the foam is fun to apply) : ) Waste is done!