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Last year I changed my bathroom vent fan to the Hengis Vortex 11 fan. It can be installed from the inside and does not require removal of your existing roof vents and covers. It works very well and has both forward and reverse speeds, great for bathroom use and showering.
There are many you tubes about the Hengis Vortex fan and its installation.
The comment about using the super fan while flushing is well taken, it can pull fumes right out of the black tank Not that pleasant an experience at all.??