Hi Basser
Some of the cabinets where screwed in from the outside in when the coach was made. To get them off from only on the inside you may have to saw off the screws.
The water damage... and the siding. It all depends on how bad the situation is. Once you have the wall panel off you can see how bad the damage is and if it can be fixed with the siding still in place or not.
Do not fear the siding. It is not that bad to deal with. If applies from the top to the bottom. So to take it off, start at the bottom and work up as far as needed.
We have another member who replaced just about all his floor and I think from inside the camper. And he did and outstanding job. See Bob's post here
Rebuilding my floor
The key is, how do the water get in? If it came thru a side seam in the front, you will have to open up the worm can some more to fix that while you are in there.
Good luck and pics help a lot in these cases so we can see what you are up against and suggest better fixes.
John
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2004 Sunline T310SR
2005 Ford F350 Lariat, V10 with 4.10
CC, Short Bed. Integrated Brake Controller
Reese HP trunnion bar hitch with HP DC