Mobilehome heater fan cycles 20 seconds off then 1 minute on?

1996 Interthrem mod MMHB-056ABHR-06 HEATER WORKS BUT TAKES ALONG TIME TO HEAT UP THE HOUSE. THE HOME HAS BEEN EMPTY FOR 3 YEARS. I CHANGED THE FILTERS AND HOOKED UP A NEW PROPANE TANK FROM AMERIGAS.