It is more efficient to create heat by burning propane rather than making electrically resistive heat by generator. If it is a matter of the batteries not lasting all night when the propane heater fans run, you need to replace the batteries with more capacity.
Your setup would benefit from a solar installation to keep the batteries topped off during the day. I have three 160w panels in the cloudy NW that keeps my batteries fully charged even if efficiency is less than half on many rainy or overcast days that are short in the winter months.
I do know some people run a small generator in very cold weather to keep the propane furnace fans going due to lack of battery capacity and solar recharging I described above. There is no reason why you cannot use your current 50A cord with an adapter on a smaller generator, but you will have to remember that it may not power your air conditioner and maybe some appliances will have to be run one at a time rather than together. I know I was spoiled with my 5500 and large fuel tank. I could run everything in my toy hauler without thought to duration or load. My thinking had to change when I switched to truck campers which run small LPG generators.