They are referring to the fresh water tank...you don't want to add antifreeze into your fresh water tank for two reasons...1) you will need to add alot of antifreeze just to get it to the point where the pump will be able to take it up resulting in using way more antifreeze than is needed to get the job done. And 2) you will have to flush and flush and flush that fresh tank come spring time to be rid of all the antifreeze in it. Are you sure you don't have a valve and siphon hose at your pump to suck antifreeze straight from the jug? For what its worth..most manuals that come with the unit are so generic that they are almost useless.
2011 GMC Sierra 2500HD
2014 Rockwood 8289WS, Diamond Pkg.