I am installing a battery in my enclosed trailer for auxiliary lighting and am curious if a battery isolator is required and why? I would like to be able to charge the new battery using the truck and want to be sure nothing gets messed up. My truck has the tow package with the 7-pin connector and 130 amp alternator. It seems to me that the isolation is already there because there is no power the the charge line of the connector when the key is off? The only thing I think I'm worried about is overcharging the new battery? I have a 135 amp isolator picked out but it looks like a lot of work to install. Any help on this would be greatly appreciated.

Thanks!

You don't need an isolator, the Tundra's with the tow package have a factory relay that shuts off power to the charge line when the truck's ignition switch is off. The normal 'isolator' that you are probably talking about also has a voltage drop accross it which reduces the charging effectiveness.
You also don't need to worry about overcharging your battery. Due to the length and relatively small gauge of wire between the alternator and your trailer battery you will never overcharge the battery, in fact it will not even charge very well. You will be far better off getting a good 3 stage charger and using that to charge your trailer batteries while parked.