Do you have Pictures/Part numbers for the separate parts you used for the drain (the actual drain and the metal union)? I need to do the exact same thing. Thanks.

Amazon.com: Valterra A01-2012VP Silver Carded Shower Drain : Automotive
Buy Valterra A01-2012VP Silver Carded Shower Drain: Sewer Hose Carriers & Fittings - Amazon.com ✓ FREE DELIVERY possible on eligible purchases
www.amazon.com
The drain on Amazon appears nearly identical to another one I saw on there by “Highcraft” that I wish I bought instead for half the price:

Highcraft 9793 Flat Stainless Steel RV Mobile Shower Strainer-Drain Assembly for Bar or Bathroom Sinks, Small - - Amazon.com
Highcraft 9793 Flat Stainless Steel RV Mobile Shower Strainer-Drain Assembly for Bar or Bathroom Sinks, Small - - Amazon.com
www.amazon.com

Halex 1-1/2 in. Rigid Water-Tight Conduit Hub 16315 - The Home Depot
The 1-1/2 in. Watertight Conduit Hub provides a rain, oil and dust-tight joint at knockouts or slip-fit holes. It connects and effectively bonds rigid or IMC conduit to a load center or other metal enclosure.
www.homedepot.com