Don't think this one out too hard. simple physics. If you can finish job and not fill tank, take back to shop and dump. I spread my jobs out so that I can stop home after every job or 2. With APO, discretely run your hose to and inconspicuous area, not near a storm drain sewer, turn her on. I'm not too cool about bringing waste hoses back into the house unless the home owner is a flaming environmentalist and insists on sanitary cleanout dumping. 40 years of this stuff will teach you a few things.
As for holding it secure in the toilet, put hose intry to get it into the toilet trap, close the lids and put a nice weight on it.
If really concerned on a job, buy a tank mounted trailer 100-200) gallons and tow it with you on those special jobs.
meh.