Don't remember the BTUs on our heater as we've had it for a few years, but I do remember it was, at the time, the biggest heater Tractor Supply sold.
I took the Mobile Home approach and provided the heater with it's own air by running the rubber hose from the 100 lb tank through a 2" clear plastic hose to the back of the heater. Hillbilly style. Granted it hasn't gotten real cold here in south Texas, but we've had several nights in the low 40's. We keep the heater on about as low a setting as it will go and it stays toasty in here.
We haven't ran the furnace even one time and the fan on the heater is a lot quieter than the furnace. I am a bit paranoid about waking up dead so I installed 4 carbon monoxide and 3 smoke detectors throughout the trailer. One has a gauge and has registered 0% and never fluctuated.
We love our heater and can't see ever using the furnace very much again. We also took Ozz's advice about wiring a switch to run the furnace fan if it gets below freezing, but so far we haven't had to use it.