The two mods I am happiest with on my trailer were the LED lights and the solar system.
One mod saves a bunch of power, the other creates a bunch of power. Silently, 7 days a week, 365 days a year without using a drop of gas.
Now me and you may use our trailers for different things, so we may have different needs. Mine gets mostly used for remote hunting trips. For me, the solar system was worth every penny as I am basically self sufficent on power generation now. I didn't spend much money on it, and did all of the work myself (which wasn't bad). But my power needs are low compared to some people. Not exactly going to be running the A/C off battery power, you know?
The LED bulb swap cut my power consumption by 75%. The LEDs are what pushed me into the "closed loop" stage. Meaning, my solar panels produce more power on a sunny day than I can use in a day, as long as I run my refrigerator on propane. It's a beautiful thing.
Don't forget the fire extinguishers! Become religious about roof maintenance, tire pressure/wear, trailer brakes and axle bearings. Winterization is vital, as well.
Actually, there is a ton to learn. And it can be a steep curve - but manageable. There is just a bunch of it.
http://www.rv.net has been the biggest help on my learning journey. Pay attention to the fifth wheel forum, because those of us with fifth wheels have our own set of challenges. Seems like 90% of fifth wheel owners drop the trailer onto the truck bed at some point, so there is a step by step process you MUST do each and every single time to make sure you don't drop yours causing $$$ damage to both truck and trailer. I haven't dropped mine, and I have no plans to. It CAN be prevented.
Anyway, I'm rambling now, good luck on your journey and enjoy your new trailer. READ everything you can get your hands and internet browser on. It can save you untold amount of money and headaches. If the trailer came with a manual, memorize it.