On the OP's latest questions:
#1 - I would run on through till you have time during a camping stay and get into town and have the code checked at an auto parts store. Most places will check it for free since they hope you will then buy parts appropriate to fix said problem. Look up the code yourself online and see what the issue / fix may be......funny'cause on our trip we just ended I left the code reader I have in my tool kit in case this exact thing happened to us...of course it did not since the tool was there.....
#2 - the sensors get crud on them and fail to read correctly. I stopped even checking the level indicator and just remember the status of when last dumped / how many days and # of people who have been adding to it....when all else fails, get out the flashlight and look down the hole - you know when it needs dumped again!
#3 - having some little bit experience with working on modern vehicles, I don't subscribe to the "try a new gas cap" thing for a check engine light. I don't know why swapping the cap would cause any problem except it probably sense the pressure change / lack of it in the evaporative emissions system and tell the computer not to allow it to run until corrected -- I'm just guessing, since I don't know the particular vehicle at all.