Yes, the water damage is usually "always" bigger than one thinks when they first start into these water-damaged campers.
You are doing great! Good job! I see you added pics too.
A heads up on making the new walls since the siding already has all the cutouts for doors, windows, etc, do a test fit on the wall wood openings and the overall lengths as you are making it. The test fit by laying a few pieces of siding over the new frame "before" you fasten the wall together helps. You can adjust easier, then. All the siding holes must line up with the new rough openings. If you can fudge a little, +/- 1/16", and even trim a little siding as needed. Try not to be off by 1/8's or larger. You can end up without enough siding to create a good seal for the window flanges etc. A router with a carbide bit and the bearing on the end will trim out the siding well once the wall is assembled and the siding stapled on. Tin snips work, too, before you staple on the siding.
Not sure how you will hold the new walls together, pocket hole screws (Kreg screw), staple, or other method?.
I created an 8' x 8' foot table top on saw horses to build the walls I do. The square edge of the plywood helps create a guide to make a square wall. I went the staple route; it works well. You have to do both sides. I bought the stapler as I do enough of these to justify the stapler. You may be able to rent one.
I also have Kreg screwed walls together. It costs less, makes a good joint, a little slower process but a good one.
Looking forward to seeing what the frame looks like and your new wall wood.
Oh, I know you stated 2 x 4's; in today's lumber yards getting good quality lumber, you can rip into smaller pieces and not have it all warped up is a common problem. Going up to 2 x 8" wide or wider can often be better wood that will not warp so badly. Look at the wood grain on the end. The 2 x 4's and 2 x 6's are cut out of the center of the log many times. That board can warp easter due to the grain. Going wider, you get away from the center of the log issue. Drying the wood is still a science that can get rushed too. Going to southern yellow pine can rip truer due to the stronger wood and maybe the better process as it costs more. The SPF, spruce pine, and fir can rip well too, but you are into 2 x 10's or 2 x 12" and at a better quality lumber yard.
Hope this helps,
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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