I sprayed them down with some water and gently lifted them up, trying not to break them apart, and sealed them up in plastic bags. 

The parcel with the arrow and above and below the arrow are ALL owned by this company.http://www.polydeckscreen.com/They are some kind of plastics manufacturer.  

So adding on a kitchen and bath may literally require whole new plumbing because mobiles used old style plastic cheap stuff that rots away and you can't fix or add toi. 

Replacing cabinets is something you can do yourself if you have the ability to get the cabinets into the space.If the countertops are a straight run, you can also find inexpensive plastic laminate counter tops and install those yourself.I hope I am not overselling something as being easy when it may not be.  

Also the blue plastic and the cardboard structure underneath it is an eyesore, I don't know what it's for but if it's a camouflage like for the trash and recycling bins I would build a partial fence structure with a opening that you can close to hide those, and to match the fence around the the property.  

Next I'd level any really bad spots and put down plastic sheeting. 

This site is my free time companion and it's always within reach for all my real estate reference.

Use plastic gloves if you do it yourself, they are toxic.

You can cut out small areas and use a product like wood filler, plastic wood, durham's water putty etc. or bondo basically to repair small areas, but it sounds like your problem goes beyond that at this point.

Plastics and rubber products do deteriorate over time.