I had to do this job on my '06 TS Insignia. The fresh tank came out very easy. Be sure to remove all lines from it including the 2 overflow lines on the pass. side. If you must remove some of the drain lines preventing the tank from sliding out the pass. side, that is no biggie, just replace using rubber couplings and or elbows as needed.
If the tank is damaged, the part numbers to replace are usually hand written on the top of tank and the mfg. is embossed, labeled, or stamped on it.
To repair a tank you need a plastic welder, knowledge to use, identify the type of plastic for use of proper welding rod. There are also some very good hot melt glues (Hot Stick Adhesives, LLC. HS-376 UltraBonder), Flexable Epoxies, and even fiberglass can be used with proper knowledge of each and their limitations.
I have done the welding and used Hot Stick glue and either would be best choice depending on location and size of damage not to mention the stress factor in the damaged area.
Goggle "rv tank repair" for more choices and insight.
As for replacing the floor, I used 1" polyboard or starboard. Polyboard is cheaper than starboard and is easy to find at farm stores as it is used in horse stalls, etc. Two layers of 1/2" poly works fine screw together using truss head screws in the areas not under tank or counter sink under tank.
Either will require addition of bracing underneath, which can be done easily with angle iron...welding is not required.
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