How do I prevent mobile home flexible water lines from freezing?
If the line is PEX, then yes, it can generally withstand freezing. Thawing is pretty easy with a hair dryer. It doesn't take much to thaw PEX. 

If the toilet shares a line with another fixture such as a sink, a simple solution would be to just open the tap and let it dribble when it's freezing out. 

But if freezing is the norm for you, then you might need a bit of heat tape. Just make absolutely sure you know what kind of tubing you have before you buy and install the tape. Only a few specific varieties of heat tape are designed to work on PEX. And those that are usually have special instructions on how to install it.