The answer on this is, 'it depends'.
If the trailer draws significant current while it's being towed, maybe
heavier gauge is a good idea. But let's say you've run your
60AH battery to 80% depth- of- discharge, and now you're
going to tow it 4 hours home. When you plug it into the truck,
it'll pull a pretty high amperage at first- and the 10 gauge wire will
limit the maximum draw. Resistance of 10ga is 1 ohm per
thousand feet, per wikipedia, so if we guess 20 feet in the tow
rig and 20 feet on the trailer, that's about .04 ohms IGNORING the
resistance of the connections and assuming a perfect ground.
Your trailer's at about 12v, your
land cruiser's at 13.5- so it'll try to charge at 60 amps, roughly,
limited by the resistance of the wire. Which should take about
an hour, and that wire is going to be toasty. Well over 100c,
per the same Wikipedia page.
But the trailer battery isn't a perfect load- it'll pull up to 12.5 volts
pretty quickly, and current drops to 40 amps. Now the wire's
just running at 90c, and the battery will be charged in about an
hour and a half.
But wait- the battery pulls up to 13v in less than an hour, current drops
to 20 a, and now, it's STILL going to take an hour plus to charge
that trailer battery fully.
An hour later, it's pulling less than 10 amps....
...and so on. Eventually, you get home, the battery on the trailer's
at 90-something percent charge, and that's your weekend.
You tow out a few hours in a few weeks, the battery tops off, and
you're a happy camper.
Probably OK. Not ideal, and especially if it's cold, the battery wants a bit more.
But, same scenario, if you only tow home for an hour, headlights, rear defrost, heater
fan on, radio, maybe some extra driving lights.... your alternator has a hard time holding 13v,
the battery only gets maybe 50% of a charge, and if you go out a month later, towing only an hour,
you've MAYBE got 70% of a charge.
So big batteries, heavy usage, short charge periods, cold weather, bad grounds-
all can add up to a system- using 10 gauge- that's a bit marginal.
My car trailer that uses the battery only for the winch and
break- away could probably function fine on 12ga, as I take MAYBE
10% out of it winching the car on and off, and tow for hours to get
to the track and then hours to get home.
If you think you're going to use a LOT of juice, heck, run 8 gauge-
but you might need some sort of current limiter to save the
system. Or just a big fuse or breaker.
I think most of us will be fine with 10.
I ran 6 to the back of the truck, and 10 on the car trailer, since
the car trailer battery will never be a 'house' battery.
But I have an Anderson connector on the back of the truck, and
a set of clips to use it as jumper cables, winch supply, etc.
It has an 80a marine breaker on it, and I've never popped it.
It's a fun set of problems...
t