General info and forum sites
IRV2 list of all their forums by category.
RV Forum home page.
Love your RV community groups are a group of forums by subject.
RV Forum - home has basic info and many forum topics and categories about RVing.

5th wheel
RV Forum - community - 5th wheels is about 5th wheels.

HDT
IRV2 - HDT has HDT information.

Manufacturers or brands
IRV2 is a group of forums, one for each of many manufacturers.
IRV2 - New Horizons brand fifth wheels.

Full timing
Escapees RV Club has a great forum full of information.
   Escapees - reasons people leave RV full-timing.
Slashdot thread about full-timing..