Please don't just leave the rot. You're a steward of your boat, there will never be any more of these Taiwanese Trawlers built, when the last one is scrapped they are gone forever. They represent an important milestone in recreational boat history - the era when ordinary people could afford one!
If you're going to enjoy it for a few years and pass it off so you can get a bigger boat you can stop reading now.
With all due respect Peter B there is no deck to cabin structural joint on an MT/CHB. There is a shelf of thick fiberglass that is part of the original pour of the two halves of the hull. There is a lip about 3" high on the inboard part of that shelf.
https://www.trawlerforum.com/forums/members/158-albums1121-picture6917.html
You can see the lip quite clearly in this picture.
In the double cabin design, the forward and aft cabins have teak planks on the inside, that are screwed into the lip to add strength. Some families screwed these through the lip and into the deck coring. The cabin walls (marine ply) sit on the outside of this lip. There's a top frame that is attached at the front and aft corners of the two doghouses. The stick frame and the walls hold up the cabins.
The main salon - on the D/C and the sedan - is held up by a stick frame that's supported by the forward and aft bulkheads, and that same lip, which is reinforced by the hollow stringer boxes that run up the side of the hull to the underside of the deck. In the sedan the forward house is built the same as the D/C.
Cabin to deck is a layer of caulk under the coring, a thin layer of chop gun glass, and typically a piece of quarter round.
When the decks go, the thin layer of chop gun goes, and the water wicks up into the cabin walls. If you crawl around the deck on your hands and knees and check the bottom of the cabin walls they will be lumpy and/or soft. The deck coring actually slides UNDER the cabin walls as the decks were cored first, and then the cabin walls were set on a strip of caulk which enabled them to expand and contract.
The decks themselves are torsion boxes, grp underneath, teak struts, and either plywood or balsa core. The teak decks are screwed into the struts, not the core. People ruin the decks by adding additional screws in the wrong places.
Adding a bunch of weight on top of the torsion boxes in the deck works, but if the core is badly saturated it is very heavy and you're adding more weight, which puts stress on the structure that supports the house. You'll get cracks in the walls as they can't stretch enough to handle the new weight. You're going to accelerate the rotting of the walls, as the original GRP shelves are going to sag under all that weight.
Amatuer repairs are often overdone and and as a result are very heavy.
The only proper fix is to recore the decks, and along the way you're going to end up scarfing new core into the bottom of the house.
If someone tells you to drill holes in the deck and squirt epoxy in the holes they don't know what they are talking about. The PO of my boat did that, when you take it apart you'll find pictures of little puddles of fiberglass floating in rotted wood.
Injecting antifreeze - advice you may also get - will slow the rot down as it kills the bacteria that's eating the wood. It won't add any strength.
Drilling holes and putting it under heat for a few months, and then laying new cloth is another approach. This works, but rotted wood then dried has about as much strength as a styrofoam coffee cup it crumbles in your hands easily.
BlueWater2 I have hundreds and hundreds of pictures of literally every piece of your boat taken apart so you can see how it is built, send me a PM.
There was so much rot in my 76 D/C that after I removed it all I raised the waterline TWO INCHES.
All this being said these are awesome boats, that can be repaired by the average guy, are surprisingly sea worthy for a coastal cruiser, and will give you years and years of pleasure at very low cost. It's hard to find a better value for the money, which is why they remain the most popular small cabin trawler ever built.