All the theoretical debates aside, there is a reason that the scientific method includes experimentation. In this case, get a bike with a power meter. Set-up pannier and trailer experiments. Repeat each a few times (or more). Then see that the actual data says.

As with most real world situations; the theoretical makes assumptions (and simplifications) that may or may not be realistic. The only definitive way to determine the answer to a question like this is to MEASURE the real world results.