I'm pretty sure all new RVs/TT/5er's must have a sticker with actual weight of that unit as it left the factory. Unfortunately my schedule didn't work out to drive up to Hershey but when I was at a show in RI last year, everything we looked at had a sticker with actual curb weight. Once you have that, just subtract it from GVWR and that is what you have for cargo.
We were looking at Class C's and I will tell you it is disappointing that some of them are only around 1,000 lbs on cargo capacity, especially as they approach 30'. Make sure it's a Ford E-450 chassis or Chevy equivalent. Pretty sure 2015 and newer E-450's have 7500 lb tow rating instead of 5,000.