This comes up a lot. I don't understand why someone with very nice onboard facilities would want to use the nasty campground bathroom either.
This past summer, I was on a caravan to Alaska. Several of the RVs used the campground showers but the one I couldn't understand was the couple with a new 40' Tiffin. They were trying to keep everything "new" and wouldn't even use the toilet! I tried to tell them, as gently as possible, that whether they used their RV or not it would not affect the resale value but they didn't believe me.
Another reason to not use public showers is no control over who enters. I was with a few of the other women in the women's shower in a park in Anchorage. We were helping each other with our hair earlier and were all in the showers when a woman came in with a teenage boy with her! Common sense is not common, but my modesty dictates I use private facilities.
And I know my shower is clean ;) I even use the one in my 24' class C.