Our A126 has the Thetford cassette. Love it. Capacity? Not sure of exact numbers but we have has 3 day trips without issues.
Besides, it's a cassette. If it gets full just walk it to the bath house and empty. Then just add the chem and your are good for 2-3 more days.
2011 FR Rockwood Premiere A126 Hard Side
TV=2011 Jeep Wrangler Sahara 2 Dr
Reese 66065 WDH, Curt WDH Shank #17120
Prodigy P2 Brake Controller, Mopar 7-pin Harness
B&M Transmission Cooler (#70268)