Welcome to the forums!! I have a 78 Minuet, be aware that the floor material in the Minuets is just a hair over a half inch in thickness. It definitely has a bit more give than the 3/4 inch floor that is in my Overlander. I guess they did it to cut down on weight. There is another forum member who has a Minuet with some sort of aluminum floor but I think it is a one-of-a-kind unit.
The soft spot by your door could be a bit of rot, this is probably the most rot prone area on any A/S. Water intrusion is the cause of most floor woes. I would do my best at patching and then do the entire floor in some sort of Pergo-like flooring, it will give it some added strength as long as you are not worried about extra weight, which I feel is negligible. There are quite a few of us Minuet owners around here so don't be a stranger.