I have both of those monitors myself, great to keep an eye on what's going on.
I also have a Kill-a-watt for spot readings, and my pure sine inverter has an LCD readout too.
I am currently working on running 4/0 gauge cable from my batteries to a distribution bar behind said equipment. Every other 12v wire that will run anything more than my
LED lights will be 8 gauge or better. I can't stand for any voltage drop with my
solar and
generator setup. Gotta be as close to zero drop as I can get.
I don't mind necking it down once it gets where it is going, as it isn't about raw amps for me. Just the resistance of the wire and subsequent voltage drop in distance.
It ends up being important when your charger thinks the voltage is one thing, but it's actually not.
Since you have the power from your vehicle going through a relay you may do well with a booster installed by the batter(y)ies on your trailer. Even 10 gauge wire will drop enough voltage to cut the charging way down from the alternator. Depending on the distance you drive at a time in hours, you could set the booster up to 15v and get a nice equalization charge from it. This will increase the life of your batteries dramatically. Just check the water level. If it's AGM 15.5 volts is better yet.