You did not say where the floor was getting soft. Reason for asking deals with how the floor got wet. The way I have seen campers get and advance water damage, you may have an active leak in a wall that may not be from the roof or is from the roof along with a wall leak.
The floor is OSB board. Basically, large wood chips glued together. It is a common building material and does well, is economical compared to plywood. The reality of it, the rating used is not meant to become wet for long period of time. The glue starts breaking down, the floor gets soft and when advanced enough, it falls apart as there is no glue left.
If you have made it to the soft floor stage, odds are favorable the floor joists under it have some level of rot too. There is a way to inspect for how big a problem there may or may not be by using a moisture meter. You can scan the floor down and then get under the camper, and scan up against the black waterproof membrane. Floor rot ends up with wet insulation and the meter can scan and find it. The meter will read about 3/4" deep into the surface so it will find wet insulation scanning up from the bottom.
You can also scan the walls and ceiling inside the camper to find water wet behind them which may be how the floor got wet. The meter will find water damage in a wall that you cannot see or smell in the living space. Water coming down inside a wall stops at the floor. The water cannot get out, so it start rotting the floor out.
This post will help on the moisture meter. This can help tell you how big an issue there may be before taking anything apart. Then you can decide what you want to do or not do. https://www.sunlineclub.com/forums/f...per-17613.html
This thread will help show you what a wet floor and walls means to rebuild. This is a total restore thread of one of my project campers. Your 1999 camper will be built very similar as far as how the floor, walls are made and how the steel frame under the camper holds up the floor. There is a lot of pics to help show all this. The floor repair is later in the post. https://www.sunlineclub.com/forums/f...avy-17684.html
If you can tell us more about where the floor is soft and where it is not, we can help better on where the water may have come from. Getting the moisture meter will tell you a lot more. We can help with how to interpret the numbers the meter is reading and what it may mean for you to expect when you start taking the camper apart.
Also, once you understand how the camper got the floor wet, the meter will tell that, odds are high there will be some level of wall repair and then the floor. The walls area placed on top of the floor. To fix these campers, most times it helps to make a better overall repair to take the camper apart from the outside, rather then the inside. This leaves the wall board more intact and less damage to repair.
Hope this helps you get started. Tell us more info and we can better help what to do, in what order and what to expect this effort may take so you can decide if you want to do this or not.
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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