As a temp/test, cover the other registers with duct tape, and make sure you get airflow to the master bedroom. If not, then you have other problems that need fixing, collapsed duct, disconnected duct, could be a number of things. If there are no problems in the duct work then the problem is, they use one size duct for the whole trailer. The farther away from the furnace you are, the less air flow you have, unlike a house where the duct work is sized down. You will have to limit it somewhere to force it somewhere else.
Could just get a small ceramic heater to supplement.
2012 Ford F250 ext. Cab 4x4
2002 Jeep Wrangler Sahara
2008 Yamaha V-Star 650 Classic
2008 Work and Play 18LT