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I have a new multimeter that I've tested the voltage of a 12v flooded lead acid battery with before.
Today I tried to test the amps running on the battery, to see if there was parasitic draw. As instructed, I disconnected the negative terminal on the battery, and connected the multimeter in series (red to trailer, black to battery). Nothing. No reading, and no power in the trailer.
Is there a way to test if I'm just doing something wrong or if the device is broken?
Most likely your DMM fuse is blown.
The better method is to put a small 12V lamp in series and check the lamp intensity. It should be very dim if you have low leakage.
If you have a resistor you can check the meter functionally. For example, a 1000 ohm resistor would pass about 12mA, and you could check it on the 199mA range
simulate this circuit – Schematic created using CircuitLab
Otherwise, check the manual (or ask here, with clear photos, if you can't find a manual) and make sure you are using the meter correctly, and if so then open it and check the fuse.
Putting the meter in current mode across something like a 12V automotive battery for even an instant will typically blow the fuse, or worse if the (cheap) meter has a high current unfused range it might cause burns by melting the test leads etc. The higher the applied voltage and available current (eg. industrial mains) the higher the risk of injury or worse.
As instructed, I disconnected the negative terminal on the battery, and connected the multimeter in series... who instructed you to do this? \$\endgroup\$