DSÖ.. In a 2018 coach, Iím guessing you have a HEPVO valve, as suggested by LtDan. If your shower floor is less than ~5 inches above the base floor, that would tend to confirm the HEPVO. DO NOT use that barbed clean out tool!
My DS is older (2002) so I donít have to deal with a HEPVO valve, but this same process will be acceptable for you. I buy two bottles of the Mr Plumber brand drain cleaner at Walmart. I use the kitchen sink as my indicator. When that gets a little slow, I clean them all. Or, every 6-8 months regardless. At bedtime I pour about 2/3 bottle down each of my three drains,ó kitchen, bath sink, shower. I leave it overnight and flush all three in the morning. This has worked for me for almost 10 years.
John, Diane, and Lincoln, the guard cat. RVM103
Fulltimers since June 2012
2002 Dutch Star 40, Freightliner, Cat 3126, 2004 Element