Here's some BTU/hr figures for a 35'x10'x7'tall trailer.
35*10*7=2450 cu/ft
Formula ( Btu/hr=Cubic Feet of air * Insulation)
Insulation = (.2 = Excellent, .4 = Average, .8 = Poor, 2 = No Insulation.)
2450*.2= 490 btu/hr per degree Fahrenheit. That's for excellent insulation. Little to no leakage.
2450*.4= 980 btu/hr per degree Fahrenheit. For average insulation, average Leakage.
2450*.0.8= 1960btu/hr per degree Fahrenheit. For Poor insulation, loose leakage.
There's about a million other ways to figure this out too but gives a general idea of whats needed.
Lets say you have average insulation. So if you need a 30 degree temperature difference then you take 30*980=29,400 btu/hr. So for 32*F - 62*F that's how many btu's you would need. I would say at least need a 18,000 Btu/hr heater.
But if you have REALLY good insulation you could get by with: 490*30= 14,700 Btu/hr.
But if it was me I'd get at least a 18,000 Btu/hr one.