Thanks for all the ideas. I'm familiar with test equipment, I just wasn't sure how far my troubleshooting might have to go. I will look at a number of these when I go out there next time. The truck has been hooked up to the trailer before, so I know that the connections worked at one time, the TT has been sitting for several months without any power (or attention). One other small point is that I happened to have my Yukon XL, not my 3500 that day, but I have towed another RV with the Yukon, so that receptacle had worked at one time too. I'd never try to tow the 311 with my Yukon, but I figured the power system should be able to light something up.
This is a great community to ask questions!