You're doing what you can to fight condensation with the plastic and as others have advised ventilation is the only way to get rid of the moisture. Showering elsewhere is a good idea too.
The hay bales are a pretty ingenious idea... I'll have to remember that. I've also heard of folks putting 2-3 100W "trouble lights" daisy-chained, to create heat under the skirted trailer.
We may be doing this ourselves next year, so let me pass on some of the things I've picked up for full-timin' here in the Midwest winters.
Note John's advice carefully... that's a 100 GALLON propane tank, which is 420 lbs... like 14 or more 30 lb bottles... not one of those little 100 POUND bottles that's not much more than 3 30 lb bottles. Keep your camper bottles filled as a backup. The gas company can probably get you hooked up. The lower you set the thermostat in the trailer, the less propane you'll use.
Don't try this without AC power AND a backup generator with plenty of gas supply. Ice storms sometimes take down power lines, and those in the country are sometimes the last to have their power restored. In really cold weather, a furnace can drain a pair of batteries in less than a day.
Remember to check the batteries frequently so you only have to add little bits of water at a time... so they won't freeze.
You can use one of those safe oil-filled electric heaters to augment the furnance. But don't let it prevent the furnace from running as often as it needs to, to keep the tanks from freezing. Remember how much power it draws (1500W ?), when selecting your generator.
You can also use electric blankets or mattress pads to allow running the trailer a little cooler at night, when it's the coldest. If you have twin beds that are up against the wall (vs an island queen), tacking carpeting or foam to the wall may help you sleep warmer.
Either a small ceramic heater or a hair dryer (keep 'em away from water!) running in the bathroom may be welcome.
Make sure your water supply faucet is freeze protected to down below the frost line. Many have a valve that drains the top part of the pipe, way down deep below the frost line, when you shut it off. Don't leave a hose permanently connected to these with the valve open. Your electric heat tape might protect your hose, but won't help the pipe from ground level down to below the frost line. Some faucets in cold country are electrically heated... until the power lines go down. Use your fresh water tank and connect the hose to the faucet only long enough to fill the tank.
Dumping the black and gray tanks may be a problem if the pipes and valves are frozen. See the UltraHeat tank and pipe heaters
for products to deal with this. The pipe heaters are 12 volts, but you don't have to wire them into the trailer batteries... you can connect them to the DC output of small generators to warm them up for a few hours before dumping.
Keep plenty of food on hand and water in the tank. It's been 25 years since there was a bad blizzard here where vehicles couldn't move, but they happen. Country roads are sometimes the last to be plowed.
I also intend to keep a small air compressor and some RV anti-freeze on hand for use in the event of furnance failure and we have to abandon ship and head to a motel. Better than coming back to frozen and burst pipes.
Anyway... hope this helps. And I also welcome more advice from those who've full-timed in cold weather.