Not one of my readers!
No toilet that's designed to flush using raw water should ever be connected to the onboard potable water system...it cannot be done without risk of polluting the fresh water supply or damage to the toilet, or both...and every toilet mfr specifically warns against doing so in their installation instructions. ONLY toilets specifically designed by the mfr to use pressurized water can safely be connected to the onboard fresh water system.So the answer to your questions are:Yes--there's a good chance of polluting not only the water in that tank, but the whole supply...it doesn't require backflow for it to happen...bacteria can migrate under any conditions. No--installing a check valve will not protect the water fresh water system. So your only two choices are:1. Disconnect all fresh water plumbing from that tank and use it only as a toilet flush water supply. Recommission the rest of the system to sanitize it. 2. Disconnect the toilet intake from the tank and recommission your whole fresh water system--both tanks and all the plumbing--to sanitize it. Directions for recommissioning are in my book...if you don't have it, search the archives...I've posted 'em at least a dozen times.