I'm new to Trulia and almost 99% a Buyer's Agent but I don't think any listing service allows one to select two categories. But I happen to have two clients who are looking specifically for rental property right now and so I appreciate your conundrum.
I think your agent is right that more folks are looking for second homes rather than pure rental plays. But my two clients are the exception to that rule, and the local MLS does not have a category for Rental Cabin.
So, my, less than perfect suggestion, is make it Residential in the MLS and Income/Investment in Trulia and Zillow (for a month at least) and see what happens..
Dixon Gary Realty