Start at the battery end with one DVM probe on the positive wire then put the other probe at the opposite end (panel end) of that same wire. If you have any voltage drop you will get a voltage reading on the DVM. You may need to make extension leads for your DVM.. Now do the same, using the same "load" for the negative lead. If both readings are the same or similar, at the same load, it is most likely undersized wire. If one has more drop than the other, eg: neg drops more than pos, and the wire gauges are the same, then track down connections & terminations.

A trick I like to use a is a constant current dummy load. This can be a car headlight with alligator clips and will give you more consistent load to compare apples to apples......

This is the voltage drop across a fuse and fuse holder. Neg lead on one end and positive on the other. At 5A it is dropping 0.111V across a 5A fuse.

Same exact set up only a 30A fuse as opposed to a 5A fuse.