Tom just look for a thermostatically controlled heater
I second Bob's thoughts.
I have an inexpensive cube heater, a yard sale item, with thermostat.
I don't recall the brand, but it isn't important, the thermostat is.
I've tried the type you describe, and they either freeze you or overheat the trailer.
The thermostatically controlled heater is very much like a central heating system for a home, just on a smaller scale.
Set it at whatever temperature you prefer, and it will keep it steady.
For $30 or a lot less, the problem is solved.
Rob
Waiting for "someday" will leave you on your deathbed wondering why you didn't just rearrange your priorities and enjoy the time you had, instead of waiting for a "better" time to come along...