I bought mine from SAMS when I bought the boat. They are decent schematics and accurate (I have a 1978 53). However, they are just schematics, not architectural diagrams of wire runs.
Some guidance...
Shore Power - 2 x 30A/120v or 1 x 50A/240v, Port and Starboard
Generator - Located beneath galley
Source Selector Panel - Select between shore (30A or 50A / port or stbd) or generator
AC Breakers - Everything (including HVAC) is 120v except Stove/Oven and Washer/Dryer. Many of the breakers are just 15A, especially those that feed strings of outlets. Specific breakers, like the stove, dishwasher, etc, are sized accordingly. There are 5 spare breakers, 3 x 120v and 2 x 240v, prior owner may have used them.
House Bank - Port bank supplies 32v
32V Breakers - In most places where there is a 120v light there is also a 32v light. There are additional places where there is just a 32v light. The water pump, toilets, trim tabs, starters, instruments (via Newmar converters) are all 32v.
Newmar Converters - there are 3 Newmar converters in the helm console and 3 in the flybridge console. These are low amperage and supply 12v to all of the instruments and instrument lights. They may have provided power to a radio, but definitely not to the old electronics that originally shipped with the boats. I removed the original DECA radar, and it looked like it would need its own generator.
Electronics - Depends on the owner, but generally these are run from the 12v battery in the generator room.
HVAC - 4 compressors feeding 6 air handlers. One in the aft salon, one for the fwd salon and galley, one for the guest cabin and crew cabin, one for the master cabin. HVAC is all 120v.
Outlets - Almost all outlets are on one of three 15 amp breakers. One for port, one for stbd and one for the galley. The appliances are on their own breakers, including the disposal. The lighting is on a couple of dedicated breakers, and the charger is on its own breaker. Each HVAC compressor/air handler is on its own 30A breaker. The HVAC water pump has its own breaker. The scheme works mostly, but it is easy to trip an outlet breaker.
The breaker panels swing open and are generally organized well.
There is no isolation transformer on my boat.
The wiring on a boat can be very convoluted, and you will simply have to follow it. Much of it is inaccessible, though the endpoints are generally accessible. Beyond the breakers there are additional fuses for the whole boat, located in the engine rooms. These are the old slow blow paper fuses, but readily available from Home Depot. For the most part, dealing with the wiring (120v and 32v) hasn't been too challenging. You will be overwhelmed at first, but after some time you will get used to it.