pianotuna - can you say the make and model of your window fan and post a picture?
My base camp trailer is a Roughneck. 2x40K btu furnaces, spray foam insulated "basement" all plumbing except tanks above the floor in the heated envelope. Very winter capable with 2 furnaces "except" only one furnace delivers heated air to the tanks compartment under the floor. So if the main furnace goes down there is no way to force warm air downstairs to the tanks while a fix is done. Either furnace will heat the trailer unless it is cold out - below -30C.
NRALIFR - I place a remote thermometer in the coldest area of the tanks compartment in the above trailer. The temp there cycles with the main furnace's cycles - but I would like pianotuna's fan setup in the return for that "main" furnace.
thanks