I installed the Flex Air on my camper as well. I was never told about the 18,000 vs the 21,000. I would have went with the 21,000 just because I always rather have a little more than less. I love mine not troubles at all. Your cost should be in the range of $800 or so. I instaled mine myself. I also installed Air Lift on truck as well. No looking back on my part. When I loaded my camper like I was going to dry dock. With fuel, LP, Water, Food, and stuff you need I was maxed right to the number. I've got a 2018 Duramax and with camper weighed right at 25,300 pound. My camper was right at 15,000. So going with the 21,000 Trail Air will not be a bad choice. Camp on my friend.
Tim & Rita Coltrain
2012 Chevrolet 2500 Duramax Crew Cab, Allison Tranny Has now been replaced by a NEW 2018 LP5 Duramax 2500 Allison Tranny.
XLR Thunderbolt 2016