I'm not aware of an overflow, other than backing all way up the toilet, then onto the floor. To fill the tank, you would fill from the top and you just keep an eye on it as it fills up.
Others have filled it up 1/2 - 3/4 then add a big bag of ice just prior to leaving for the campsite. Then it swishes around in there knocking the crud off of the sensors and walls as you drive. I'd have to search it up, but I think they add something like a detergent to help clean stuff up, too.
John & Jennifer and family
2018 Jayco White Hawk 31BH 34'
2004 Surveyor by Forest River 29'
1996 Coachman Catalina 25'
TV 2010 Dodge Ram 2500HD SLT 4WD SB CC
2002 Jayco 12UTD PopUp