Here is a link to the WFCO 75amp converter manual. These models all use the same location for the Reverse Polarity fuses and depending on your model they use different fuses.
They may not be marked as reverse polarity but they will be side by side where the 12V connects to the converter and will either be 35 amp or 40 amp.
As has been said they are there to protect your converter in the event of an accidental incorrect hook up of the batteries.
LINK:
http://www.wfcoelectronics.com/Image...rDocs/13-3.pdf