We've been renting our Sprinter van conversion out for 3+ years now and have had fewer rentals this year - part of this was due to having the van off during some body work, but considering that it still seems low. We figure between the prices of diesel and the economy it is probably normal now but was curious if others see the same.
Try checking your local competition. Do an outdoorsy search for similar rigs near you, check prices and how many are listed as available during a weekend in the near future. This should give you a general idea of how popular things are, and what your prices should be.
We noticed this in our area, and I had to adjust our prices down a little more than I normally do during the winter months. That did the trick though, we are now booked out through December, all of them 1-2 week reservations.