We use a Honeywell Cool Touch Oscillating heater in our little hybrid. The last weekend we camped, it got down to the low 40's and even with a canvas bunk end we stayed completely toasty warm inside. That heater is especially nice since it can oscillate and blow warm air across almost 180 degrees.
In a larger trailer you might need more than one, but at long as they are on different circuits they should run just fine on 30A service.