Any recommendations as to what kind of cleaning solution is best for cleaning upholstery? I'm planning on using the portable stair tool for this.

Thanks!

I use whatever is in my sprayer at the time - usually
Bioforce's Outsolv, Challenger's Orbit Natural, or Vacaway's Encap-Green.

And make sure you dry-vac everything first. And dig out all the stuff that often gets lost way down in the crevices. Do this with some caution, as you don't want to encounter anything pointed or sharp that could cause injury.