I'm looking for a company that can write a commercial policy on a
vehicle (an RV) only during the few weeks out of the year when it is
used commercially (not a predictable schedule).  As far as I can tell,
such a policy doesn't exist (?) so I'm wondering if there is anyone
who writes customized policies.

You won't be able to purchase insurance for only a week at a time.
You'll need to buy an annual policy from your local indedepent
insurance agent.  They'll write the RV on a commercial auto policy. 
The reason that you can't just pick and choose when you have coverage
is fairly simple--there's always an exposure to loss with a vehicle,
even when it's just parked.  If you're having to provide a certificate
of insurance to a lender or a third party, you'll simply have to
purchase an annual or semi-annual policy.  Progressive will write this
on a commercial policy for the best price.

Thanks feuerbach - I've been waiting a while to see if someone could
come up with something for me.  What I was hoping to do was keep
private owner insurance on it full-time (when it was in storage or not
used commercially), and add the commercial coverage only when it went
out on rentals.  Would that make any difference?