I have excellent literacy skills even if my parents didn't sign, and I didn't hear well.
Higher literacy skills hardly depends on parent's ability to sign- it depends on their general attitude toward education per se.
If to a parent an education, literary skills, matters - then it reflects on their children.
The majority of the deaf parents I knew were themselves hardly educated. They had poor literacy skills, and weren't involved in their children education either.
I remember an instance when a hearing child asked his deaf parent for healp with basic math. the parent didn't learn math himself either so he just shrugged: "I dunno math, I can't help, sorry". he didn't organized any help from other sources either (tutoring, after classes homework club etc). So the child eventually failed math. same with English: "I dunno, I'm sorry you have to it yourself.."
When I pointed out to one such parent that their child is falling behind in early elementary grade, their response was "she is just not interested in this". Whewn I pointed out the child needs and education to get ahead in life they just shrugged "what will be will be..".
No concern, no worry, no parent involvement.
The same applies to uneducated hearing parents to whom education is not important. They do nothing to help their children, deaf or hearing, to succed in school. No matter how well they will sign, it won't help their kids to achieve good literary skills.
Fuzzy