MPG 184 has soft spot on floor right inside the door

[Barefootdiver2000]

We bought the 184 last fall and didn't notice the soft spot in the floor right inside the door (former owner had a thick rug over it when we looked at it and we missed it). Before I tear into it, does anyone know where the water might be coming from? It rained hard this weekend while we were camping and didn't see any leaks, so possibly the toilet or kitchen sink leaking from underneath?
What is the best way to tear up the floor? Just cut around the vinyl or take out the kitchen cabinets and possibly bathroom wall?

 

[Barefootdiver2000]

Here are a couple pics of an access panel that looks like was cut into the wall (not by the factory). I assume there must of been a leak from the toilet that was fixed. Not sure why they would add a shut off valve?

 

[david-steph2018]

First find where the leak is coming from. You will need to determine the amount of damage so you can have a plan to fix the damage. Since the kitchen cabinets are at the site of the damage, you may be able to get into the bottom of the cabinets to see if there is any damage there.