Think in terms of the length of the vehicle plus the length of the trailer plus the round about path. Double that for the collective B+ and ground. Then measure the current consumption with all trailer lights active. Consider the wire gauge along the way, which might also be inconsistent. Coinsider all the connections. Crimps can be serious sources of resistance. Again, with this data a substantial voltage drop doesn't surprise me. The higher the current draw (with all lights plus a few additions) will expose the limitations of both wire gauge and connection quality. A bad connection may not show up using the minimal current used in a multimeter to check resistance. Problems are exposed with a higher current draw and especially measuring voltage when the circuit is loaded. Measuring the voltage drop is the most effective method. You just have to use a little logic to narrow down the specific cause or location. It's very simple if we don't complicate it.

David