...So, I need to find out exactly how much juice those farkles are using.
My questions is: where do I use my voltmeter/ampmeter to check?....
Unless you have a muy bodacious ammeter, ya kinna doo it, lad. If you do have an ammeter capable of reading at least 10 amps, you could put the ammeter in series with the positive battery cable.
The average Volt-Ohm Meter you get from Radio Shack will only have a milli-ammeter (reads in .001 amps, might max out at 250ma, which is a quarter of an amp) built into it.
All is not lost, however. You can use the formula P=IE to figure out the current that is being drawn by those farkles. P is the Power in watts, I is the current in amps, and E is the voltage.
You should be able to find out what the power consumption of the farkles are and then calculate the current draw by flipping the equation around this way: P/E=I.
If the grips are 40 watt items, then the current is 40W/13.5V=3A. 13.5v is about right for the electrical system. Also bear in mind that, if the grips have a decent controller, they are not on all the time.
Hope that helps, though I bet the manufacturer actually lists the amperage each item draws... Lights, of course, are rated in watts.