This kind of work is really not hard to do. It took me about 1/2 hour start to finish (I already had the hole from the removed AC). Just plan it, and do it! The key is to use a good polyurethane (3M or Sikaflex) and have the mating surfaces clean.
I would think twice before I put a Fantastic fan vent in a bathroom. They are fantastic, but even on the lowest setting there is quite a lot of airflow. Unless it is an unusually large bathroom you might find it sucking the toilet paper out the ceiling! If you only open the lid just enough to turn the fan switch on, it cuts the flow a lot, and that might work (or shroud part of the opening). I have the crappy fan in the bathroom as well, but haven't changed it out. I did in my Bigfoot 5th wheel (with a relatively large bathroom) and almost regretted it. I wish Fantastic made a bathroom vent.
