I don't need to know! I use the Henry Ford principle in cases like this
So you dont know? so why make statements about something you do not understand? you said "Probably the easiest way to keep the trailer battery topped up",
I think to most people that implies a constantly full battery.
If you were to measure the voltage from the Alternator and then measure the voltage that actually reachs the teardrop battery, you will see a big drop (over 1 Volt lower)
Now given that a standard alternator cannot fully charge a single battery when all the charge is applied direct only 2 to 3 feet away, how can that same output (fully) charge a second battery that is many feet away having lost around .8 Volt minimum just via the isolating diode and more via the long length of wire to the second battery.
Nothing at all wrong with keeping it simple, as long as simple works well.