I feel it is wise to think about voltage drop differently when looking at recharging batteries, particularly from solar. The acceptable 3% for home & industrial wiring means the incandescent lamps will be a bit dimmer if they are on a fully loaded circuit (something that is rare). Under most actual conditions home wiring (as well as the 12v wiring in your trailer) are not loaded anywhere near capacity, so the real voltage drop will be far less than the calculated.
The exceptions? Because of the cost & efficiency, inverters are often sized as small as possible so they very often closer to maximum load for wiring capacity. Since many have built in electronics to disconnect them when the battery voltage drops below a set point, oversized wiring is necessary to prevent voltage drop in the wire from looking like a depleted battery and shutting down the inverter before the battery is really down.
The other exception is the wiring from a solar panel. At the time you need it most (bright sun) you will be drawing continuous calculated amperage. If you use the standard tables to determine wire size you will be losing unacceptable amounts of the panel output to heating the wiring.
In the case of a solar charging system every amp hour you put back into the battery is less time you need to run a generator
, or the lower wattage solar panel you need to replenish your batteries. While you can certainly go overboard with wire size, every percentage of voltage drop is costing you amp hours getting back into the battery. You only have so much sunlight...
Panels are getting more efficient and less expensive, so I suspect it won't be all that long until just putting a higher wattage panel will be less expensive than going to oversized wire, but I would shoot for less than 1% for a current installation.
Although he does go on, if you want some interesting opinions on RV solar installations, try reading Handy Bob's Blog
. A bit of overkill for the shorter wiring distances in our smaller trailers and lower power systems, but good stuff.