Our 2017 21RBS had no filler opening.
After two Jayco 197's, it was a bit of an awakening. Filling through the overflow/vent is about the only reasonable way, I guess.
I solved the "Oh bleep! No water here?"
problem. I carry a plastic upright 20 gal(Cdn) tank in my truck and have it plumbed so I can hook a garden hose to it to fill and also hook a garden hose to another valve and pump water to the trailer with a 12v whizzer pump. Works like a charm. I carry a 12v battery at the back of the truck box to power the little pump. It lasts a long time on a charge.
I also carry a 45 gal DEF barrel and a macerator for emptying the grey and black water tanks. I made up a cord from the two deep cycle 12v trailer batteries to run the macerator and pump water from the truck tank for the macerator cleaning. It all works good and is simple.
I have a cap on the truck so none of this stuff is obvious. I had to pack up one time and drive 30 miles to dump the tanks. Last time that happened!