I'm using
this style of heater, they say 1500W but they really draw ~1,000W on high and 500W on low according to my killowatt and smart plugs. On a 30A you could run 2 on high easily enough. If it gets really cold they might or might not be enough to keep it comfortable, you might want to have a buddy heater available for the very coldest nights. Also note they'll do nothing to keep your tank warm, my trailer was winterized back in November and I've been carrying jugs of water in and using the facilities in the house.
As far as temps, 10F is the coldest day I've seen without any propane use, that was after I got the 3rd heater. My trailer stays at 45 at night and then starts warming up to 72 90 minutes before my work day starts, 30 minutes before the propane furnace is set to 65, so it only comes on if the electric hasn't raised the temp from 45 to 65 in 1 hour. At anything below 10 degrees that was happening most days with 2,000W of electric, like I said with 2,500W it's only happened once that I've seen.
Basically at least in my trailer with Reflectix in the windows and the vents stuffed with Camco reflective pillows I'd be fine down to 10F easily, below that I'd want a supplemental source of heat.