I agree with Mr. Tingvall, you may have a right to sue the seller, who also appears to be the landlord, for breach of warranty arising out of the sale. Unfortunately, a lawsuit over that issue may take some time to improve your situation. You may also have a claim against the landlord for failure to carry out his duties under the mobile home landlord-tenant act that could give you some more speedy relief.
Under the act, your landlord has a duty to protect you from adverse effects of water on the premises. RCW 59.20.130(2). To trigger your remedies, you must give written notice of the problem to your landlord, who then has 30 days to commence remedial action to fix the issue. RCW 59.20.200. If he does not fix the problem within a reasonable time, you can get bids to have the problem fixed and deduct the cost from your rent or sue for rent abatement until the landlord fixes the problem. RCW 59.200.210-.220.
I recommend you take both approaches to get the problem fixed as quickly as possible and to get any compensation you deserve.
Did you receive a seller disclosure statement (Form 17) before you bought the manufactured home? The seller disclosure law was amended in 2003 to require the seller of a manufactured home to deliver a disclosure statement. RCW 64.06.050(2)(d). If you received a disclosure statement, the seller should have disclosed the flooding issue in section I.7.A of the Form 17,
Regardless of wehther you received a disclosure statement, the seller should have disclosed the flooding issue, if the problem existed at the time of your purchase, the seller had knowledge of the problem, you did not know about it and the problem substantially adversely affects the value of the property. Of course, you should consult a real estate attorney for more specific advice.