The 2020 Prestige models 29MVP, 30XP, and 31FP were all listed as having a 47 gal fresh water capacity. Less 6 gals for the water heater leaves you with about a 41 gal capacity in the tank. Maybe a little less depending on where the overflow was placed.
You could meter the water going in, or measure it in buckets as it drains.
Linear Mode
