Plumber's grease- oh that's why !
I'd read online about using vaseline on my toilet blade seal to keep it supple and nice- and it holds water well--- well until it got hot!
We are in Phoenix and with the fantastic fan going the van will stay close to ambient temps but when those temps are 110º that's where the difference happens.
at some point the seal allowed my inch of sitting water to drain through over the course of a week.
I was doing some plumbing in the house last week and grabbed my tools and supplies incl. plumber's grease- I looked at he label and realized that the grease is good to 180º...at there lies the diff, at lower temps vaseline will work fine, but at the high temps we have here, it just turns to a liquid and flows off the seal.
So I am now fully greased up and that problem has gone away.