I'd suggest you guys start off on your journey installing a master disconnect switch if you dont have one already, its the amps that hurt/mame/kill and your battery bank <--> fuse box is the big source of power thats hard to shut off.. you can shut off engine and unplug from shore power easy and pull fuses on 12v side, but disconnecting battery cables can create a spark and they tend to be heavy duty enough that they can be hard to bend out of the way.. once thats on there you can confidently work on wiring, adding new circuits/fuse boxes, etc and plus you probably want one anyhow for storing your vehicle and not worrying about parasitic drain.
Ive completely rewired my trailer since getting it a few months ago, battery disconnect was first thing I did.. then I put new batteries in, new fuse panel/charger in, low voltage disconnect to protect my new battery, battery monitoring system, hardwired ac/dc fridge, 120v wall mounted heater, bunch of LED lighting, a few radios.. all while planning on a rather large solar setup and inverter w/transfer switch down the line.. man I'd hate to think what I'd of paid someone else to do all that rather easy work.. wouldn't of been cheap.
If there's an art to any of it, its wire routing/pulling.. this is where experience comes in very handy, if you simply cannot figure out how to get a wire from A to B safely and out of sight then look in your local classifieds for a low voltage contractor.. they have the tools and experience and you can usually get any wire ran to anywhere you want, terminated on both ends for $100-150 nice and easy.. its paying someone to design the system and hookup all the wires that will start to get out of hand when you have alot of electrical work to do.
Id be weary about car audio installers, im sure there are some good ones out there but damn ive seen a ton of hackjobs performed too.. they just know how to hide the butchering where the customer never sees it.