I have an original owner's manual which includes a very basic schematic. It does not include a wiring or plumbing diagram. I can take it to work next week and scan if that's what you are looking for. Send me a PM with your email address.
There is a paperback book published annually with floor plans for many current and older US powerboats. I think it's called "Powerboat Reports" or similar. If you can find a copy you may find a floor plan for the Viking Motor Yacht.
The publication is called Power Boat Guide, and the boat is in there. It doesn't allow printing or copying on my digital copy or I would try to post it. Brian
Have you tried to call Viking or Lazzara? Lazzara was formerly Gulfstar that built your boat for Viking.
Here you go, click the dropbox link for high resolution version. https://www.dropbox.com/s/3er7bprjjgjw9cc/IMG_8923.jpg?dl=0
That is really putting a lot in a 55' SF. They don't put 4 staterooms in a 55 today. No aft staterooms.
Thanks for the photo. Still a lot of boat and Viking doesn't build anything like it today. That's not meant as a criticism of what they do build. Frankly, I'm not a fan of an aft stateroom but many are. I am a fan of Viking.
These were actually built by Gulfstar for Viking as were all of the motoryachts in that era (with exception to the smaller aft cabin MY's Viking did themselves like the 44'. Gulfstar then became what is known as Lazzara and Viking picked up Princesses re-branded as Viking Sportcruisers. All around 1999/2000 or so.