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So, I'm not sure whether you are planning to "strategically default" on your "upside down" home and let it go to foreclose, OR you are hoping to hold on to the property and rent it while you down size to a manufactured home.
If your intention is to "default" I HIGHLY recommend you speak with both a RE Attorney and a CPA to mitigate your legal and financial damage of doing so. Do not use word of mouth to make these decisions, nor should you follow the advice of Realtors on this forum for guidance on this subject matter.
If you will be holding on to the property and downsizing, you have additional options other than a manufactured home that might work better for you. These options come via the IRS's definition of what constitutes a "Qualified Home", as follows:
"For you to take a home mortgage interest deduction, your debt must be secured by a qualified home. This means your main home or your second home. A home includes a house, condominium, cooperative, mobile home, house trailer, boat, or similar property that has sleeping, cooking, and toilet facilities."
So, based on what you have shared ("a town that is slowly going downhill") it appears the area you live in is trending downwards. Perhaps a mobile home or house trailer would better meet your needs at the present time and allowing you to travel to other areas to hopefully enjoy your retirement more fully.
If you would like to post more details about your situation I might be able to identify other options.
There are companies that specialize in loans on manufactured homes. Not all lenders do these loans. Be aware that you will pay a higher loan rate. 1% or more is common. Also, you may need as much as %50 down. Also, there are rents for the land that the home sits on. Have someone run the numbers and make sure this is best for you. Below are some companies that do manufactured loans. I have not used any of these companies. Generally mobile homes are not considered great investments for the purchaser or their heirs. I realize that may not be your goal. I just wanted to make sure you are aware of it. One more thing. Don't limit yourself to lenders in Sonoma.
I would suggest contacting a mortgage banker because there is help out there, you just have to find.
Traditionally credit unions have been more lenient, however, banks do provide programs that may help as well.