If yours is like mine you can't gravity fill it. What most folks do and I'm planning on doing it pouring the prescribed amount of bleach into the hose, connect the hose to the trailer, then connect to water and turn it on. The water will push it in, then rinse it off, so don't worry about straight bleach hurting anything, it will be diluted before it can do any harm.
Now-2014 Sierra 346RETS 5er BUB
Then-2002 Keystone Springdale 286RLDS TT
Nights camped in 2014-28, 2015-127, 2016-10