You should really have the circuit tester pictured above or a hand held multimeter. A multimeter will diagnose many electrical problems besides just measuring voltage. An inexpensive meter can be purchased at most hardware and big-box stores.
I'd suggest to have a structured approach to your trouble shooting. I would start at the source connection (your home's receptacle) and test at every connection following back to an interior receptacle in the RV. 120V will operate devices with neutral and power phases switched so just plugging in a lamp or motor will not correctly diagnose the problem.