Any "simple plug in" ammeter that would give you total amperage at the power cord would have to be installed either at the campground pedestal or at the coach end of the power cord. If it were inside and plugged in, it would only give you the amperage use at the plug where it's installed. To my knowledge, the only way to get total amp usage "INSIDE" the trailer would be with a "hard wired ammeter" since there is no plug at which you could install the meter where it could measure "total electrical use" except at the power center (on which there is no plug to use for a plug in meter).
You can purchase a "simple plug in" surge guard with LED display that has the capacity to read line amperage. These devices plug into the power pedestal and the RV power cord plugs into the device. Here is one source to the surge guard owner's manual. In it you can read the capability of both the 30 amp and the 50 amp devices and exactly what amperage readings you can get from the LED display:
http://www.campingworld.com/includes...r%20Manual.pdf