There is a reason why the heaters come with a short duct and maybe a T. The little fans in them might not be able to push through 20 feet of ducting and 4 Ys. My owner's manual for the Espar D2 had a diagram that showed the suggested limits on ductwork. Total length and number of elbows, Ts, and Ys. It was surprisingly short, like two Ys, three registers, not to exceed 12 feet total. The reason is that the little fan has to effectively cool off the heat exchanger. Too much resistance and you'll get overheat errors, sooting, and short fan life.
You are most likely to be happy with Ys instead of Ts (much less resistance). Don't get the Ys that have internal dampers used to control the air direction. Too much constriction. Make your most important register your first register (usually the main cabin). My final run is about 6 feet. Instead of using the aluminum/paper duct that is generally purchased, I spliced in 3 inch PVC pipe. Cheaper, smooth wall, and good to 160 degrees when used for water plumbing. For hot air, PVC pipe can deform above 210F. Should never be a problem with the heater. Elbows and Ys are available at any hardware store.
Bottom line, it could be that instead of 4 Ys and longer ducting you need two heaters.