Welcome! From working on my '79, I can definitely say you'll not have much left if you gut the trailer from the inside. It will literally have the integrity of a big cardboard refrigerator box.
All the cabinets, particularly the upper cabinets, are screwed in from the outside. To remove them without destroying the walls, you'll need to work from the outside in. If you have a lot of hidden water damage, this is for the better.
I've discovered in these older Sunlines, the walls really don't have proper framing. They are more like 1/8" paneling with support boards glued/stapled to the paneling to add rigidity, and then those support boards are stapled to each other to hold the whole thing together.
Like John said, the best news of all is that it can be completely rebuilt. I don't plan to reuse much wood from mine, except interior cabinets. Most of the parts on these older Sunlines you can still buy today- roof vents, lights, exterior components, appliances, etc. You definitely bought it at the right price where you can still restore it properly and not be too deep into it.
Do you have any pictures of it?