The advantage to using electric vs propane is is you save your propane, and use electricity you are already paying for provided you are in a campground. If you are parked at home and using your own electricity then it's probably a saw off for the few nights you will be having to do it.
Yes the furnace will send heat to your storage areas which a space heater will not do, but at 25 degrees I don't think you need to worry about that too much. As long as you keep it above 32 degrees you will be fine, no need to keep it hotter than that. No need to be concerned about your black and grey tanks. 25 degrees won't hurt them. Since your holding tanks are not under pressure and subject to expansion which is what water does to pipes, no need for concern. We camp often in late fall where the nights will get to about 25F and with some heat on in the unit, everything is fine. The temps likely only get to that stage for a very short time and as soon as day breaks and the sun starts to shine the temp gets back up to above freezing quickly. If you are camped and gets that low you may want to shut off and disconnect your fresh water supply so as to avoid having that hose freeze.
2011 GMC Sierra 2500HD
2014 Rockwood 8289WS, Diamond Pkg.