California Department of Housing and Community Development provides Mobilehome Ombudsman services. They can legally assist you with questions or complaints on issues of mobile home park health and safety issues such as unsafe electrical, sewer, gas or water systems. I don’t know if a failing shut off valve would come under that category as it is not an immediate danger to health or safety.
The Ombudsman CANNOT Assist You With Unfair or illegal management practices by mobile home park management. The Ombudsman cannot mediate or offer any legal advice on mobile home park rent disputes, lease or rental agreement disputes, or similar legal matters. They tell us on their web site
that you should seek legal aid or assistance from an attorney or your local government officials in these matters.
It is a fine line but it seems like they are very limited in the scope of what they can step in on. A shut off valve is a VERY inexpensive item. In many parks the shut off is considered part of the home and the home owners responsibility ... Just like a plumbing problem inside would be. Even if this is clearly the parks responsibility, this is not an issue that I would want to go to war with my landlord over... It is just not worth you time or aggravation.
The parts will cost about $5. Installation is very easy. You can leave the old one in place and install a new one under the home where the water line comes out of the ground and into the home. If you can not do this yourself ask anyone that has any handyman experience. The entire job should take less then ten minuets.