A home that you live in is not considered countable resource. If you have a second house then that is considered as a resource. If you were to live in a house/home that is not your property and you do not pay fair market rent, it is considered in kind support and maintenance and can actually lower your SSI benefits.
http://www.ssa.gov/ssi/text-resources-ussi.htm
http://www.ssa.gov/ssi/text-living-ussi.htm
This raises an interesting question. Is the motorhome going to be used as your home or vehicle? Mr. Pham is correct on the home. You are also allowed one vehicle. If the motorhome is a second vehicle, then you'd likely lose your SSI if it is worth more than 2k. But if it is your only vehicle, then I can see where an argument could be made for it to be excluded.