donn0128 wrote:
I see no problem. Thetford and others make a "Y" exactly for that purpose. Heck, even high end fivers do this, especially rear kitchen models.
The "problem" here is that I seriously doubt if both tanks are at the exact same level. If you connect them and the tanks aren't at the same exact level, then the water from the higher tank will automatically go to the lower tank and fill it first. If that tank is the shower tank, the water might back up into the shower before it can fill the higher tank.
Ron