I was denied visitation due to pending charges. I was told I can write an appeal letter to the warden. What information can I provide to the warden that may help me with being granted visitation despite the pending charges?

This is a common occurrence and you will need to give some solid reasons for visitation privileges because the big rule for inmates is "no contact with known felons". You should address your crime, whatever the circumstances, you must show remorse and ask for them for forgiveness. The warden is essentially the judge here and if they feel you are bullshitting, they will deny it without reason.