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At 120 volt, 12 AWG wire is a good size and will handle a decent load. At 12 volt, 12 AWG wire is not good for large loads at all.
To run 13.3 volts (approximate output of an alternator) 40 feet at 25 amps with up to 3% loss acceptable would require 0000 AWG wire. If you want less than 3% loss, then even 0000 AWG isn't beg enough.
With 12 AWG wire running 13.3 volts 40 feet with 1% acceptable loss, the most amps you are going to push is 0.25. So yes, over time you will be able to send a charge to the trailer, but it will not be very much. It will be enough to put a small charge into an idle bank with no draws on it or keep a fully charged bank with basic parasitic draws topped off. It is not something you want to rely on to keep up with a heavy draw while driving.
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