Question about submersible heaters for aquariums.?

I live in a trailer, and have a 10 gal tank with a betta, and a yellow danio, and a neon tetra. I've had them for going on two years, in the trailer. Last year I could afford to heat the trailer, so the fish were warm and happy. This year, I got laid off a month ago and can't afford propane to heat the trailer or an electric heater. So I bought a Tetra Submersible Aquarium Heater (50 watt, up to 10 gallons) thinking that would keep the fish nice and warm during the icy cold nights (it got down to 30 degrees F last night.) Well, when I woke up this morning, I looked at the thermometer and it read 69 degrees F. CRAP!!! all the fish are still alive, but the 50 watter is obviously not going to cut it. My MAIN question is, do you think that if I took it back and bought the 100 watt heater (up to 30 gallons) it would keep my aquarium at the optimum temp of 78 degrees F? Both of these heaters are equipped with an automatic thermostat that turns on and off keeping the water at 78 degrees. It's just the 10 gal one isn't powerful enough to keep the water that warm when the room air is almost freezing.