Here are some numbers from the electrical engineering book;
10 gauge wire has a resistance of 1.018 ohms per 1000 feet.
Your 120 foot run should be 0.12216 ohms for each wire, so, you are correct, there should be only a few volt drop, not 12 volt drop for a 14 amp draw.
To get that 12 volt drop, it's behaving like 14 or 16 gauge wire.
I suspect the copper wire was nicked or partly cut when buried, or does not have the circular-mil area in its cross-section that it needs to have to be true 10 gauge.