If she doesn't have enough work credits (quarters worked), then she won't qualify for SSI. But if she's already getting SSDI, she must have qualified/had some work history to claim disability from that past relevant work. You should ask the SSA about this, though. You can start doing some of your own research/math:
https://www.ssa.gov/planners/credits.htmlIf the sale of the trailer involves having savings, that might affect her SSDI. Any way she could transfer the trailer to be in your name now, and then you sell it once she gets into subsidized housing, thereby any income from the sale is yours and not hers?
I would also consult a lawyer about this; I know there are ways to set up trusts for disabled family members that keeps their assets from affecting their insurance and other eligibility.