I am going to guess that the two batteries in the trailer are wired in parallel as a single battery bank, and can be treated (for a simple system) as one very large battery.
The reason for asking how much current is being drawn off the batteries while they are being charged is that what ever this current is directly subtracted from the charging current. Say that the trailer has a fridge in it. That means that the trailer battery(ies) have current demand of somewhere between 2 amps and 20 amps and those amps will not be used to charge the battery(ies).
The 7 pin trailer light connectors have a terminal for charging any trailer battery. This is a quite common feature of the larger RV type trailers. The problem with using this method of connection is that the terminal size limits the current to about 10 amps maximum. That is normally OK, you just won't be self jump-starting through that small of a connector.
The diode bridge type of isolator can result in poor battery life unless equipped with a voltage sensing terminal. Diodes have a "cracking voltage" which is the minimum voltage that has to be applied for them to turn on. This voltage is lost, it never makes it to the battery(ies), which reduces the maximum charge potential. In general more sophisticated battery management has moved away from using diodes.
My own preference is to use a "Voltage Sensing Relay" also known as an "Automatic Charge Relay." These devices sense primary battery voltage and allow it to be recharged after starting before connecting the trailer/auxiliary battery(ies) to the alternator.
http://www.delcity.net/store/Voltage-Sensing-Relay-!-Single-Sense/p_793852.a_1
http://www.westmarine.com/webapp/wc...rue&storeNum=5002&subdeptNum=9&classNum=12106