Thanks Everyone,
I never thought about liability, not that the high stop light matters much with the travel trailer behind it.  I saw on one of the cap websites a reference to a university study that showed an average MPG gain of 1.5 with a cap over an open bed.  Takes a long time to justify a cap based on that alone.  I tend to tow fast, I work a lot and get precious little time off, so when I have time off I tend to want to get there.  I'm typically towing at 75 MPH so aero will matter.  If I towed at 60 I'd gain 1.5 MPG or more, well maybe when I retire.