First, what is the deck made of: redwood, cedar, or PT wood? I always tell customers that no need to put any sealer on until the wood totally dries out. Here in Texas during the summers, that should be about 60 days. Then I tell them that sealer is not needed anyway unless they just want to do it. These three lumbers are ok for outside left alone.
I always recommend using CWF-UV from the Flood Company, or any exterior wood treatment from the Wolman Company. Thompson's is not any good at all.
Your steps. How long are the steps in lenght. I always use one stringer every three feet max. I sister the stringers to the deck end board and attach them to sunken posts in the ground on the front. then I attach each stringer to one another with a 2x6 to stop side ways swing. I always have my treads out of 2x6 material. I attach supports to each stringer from the bottom of the tread to the ground for additional support.
I imagine that you need to add supports like in exterior walls in your home called fire supports. this brings everything together.