In relation to the trailer plug, I would just solder on an extension, but stagger your joins so they are not all together as it can make the loom very thick in one spot. Just leave the joins and inch or two apart and use heat shrink over each join.
About the two plugs, I have just made up adapters, get a male plug for your small 7 pin with about 6 inches of wire to a female socket of the large 7 pin.
For battery isolators, I use the Piranha brand and haven't had any trouble in the last 15 years. I have the older 150amp that I just pulled out of my troopy and installed in my Disco and is working as normal 
I also have one of the new 180 amp units in my bush truck.
There are so many it is totally up to you
Good luck and merry xmas to you and yours
Neale
85 Range Rover Ute (Project in pieces)
89 Range Rover Classic (Black Thunder)
93 200tdi Disco,(OGRE)
96 300tdi Disco, DEAD MOTOR
04 Nissan Patrol with ALL the fruit
09 Cub Daintree Kamperoo
12 VE II Commodore Ute DD
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