You will not have problems with your tanks freezing, or any inside water lines for that matter if you use your space heater. What i would do is ensure you keep your fresh water tank over 1/4 full. same with grey and black. That much water takes a long time to freeze. A simple experiment is fill a big bowl of water and small bowl of water and put in freezer. Notice how the large bowl of water takes a lot longer to freeze. So i would not dump tanks until near full, and keep lots of fresh water in the fresh tank to be safe.
The idea's of the fan on return air are great, If you are too lazy or dont want to do that you could once a day turn the propane furnace on once in morning once before bed and let it run (and circulate air) into the tank areas for 10min.
Ive had a northwood RV for over 5 years and they are so well insulated i have never even worried about anything freezing and been good!