Installing a DC to DC converter

I'm looking for someone who has installed a DC to DC converter in their tow vehicle, in order to charge their trailer batteries using the trucks alternator.  I have a Ford f-150. It has a 200 amp alternator. 

In order to determine how much excess amperage I have available from the alternator, I was going to put an amp meter on the output from the alternator cable, and turn on the engine, turn on all the lights turn the radio on, turn the air conditioner on high. And I figured that would give me a good reading on how much it's drawing. I would like to get a 60 amp converter. Right now I have 265 ampere hour batteries, but I intend to upgrade to twice that much or three times. 

So I'm looking for any kind of feedback, any advice, any opinion on whether these DC to DC converters actually work and whether they do a good job charging the trailers batteries when you're driving.

Thanks

Harold