Google "100W resistor". A bunch show up. Even Walmart has one! Don't know what will be easiest to get in PR. Buy two of them.
Set up the panels, if possible, so that you can test their output at full sun with the panel straight on to it. Then angle it off at 45°s and check the output again. Crude, but it'll give you a less than optimum output for comparison.
Monitor the voltage across the just one resistor as the load and, of course the current through it. Do the same again with both resistors in parallel (and the in series, if you like). We're just varying the load in an attempt to get near 13.5VDC optimum charging voltage). Use the resistor arrangement that best meets that goal for your tests.
Understand, of course, that the panel manufacturers rate their products at a perfect, noon, straight-on time. And a sea level, output is at it's least (compared to 5,000 feet, for instance).
Let the owner know just how much the panel(s) output varies through the day and with alignment. At best you can only demonstrate a crude average. Probably close to the 2.5A your seeing now (approx. 30W)