What's the small reservoir behind panel over toilet?

Thanks in advance.  I am not sure what this small white reservoir is, once you unlock and open the wall panel above the toilet.  I am wondering if this is where I put sanitizer?  I'm having some learning curves here.  

WINTERIZING: My 2014 CS-S has its line drain valves in the rear, under the sink and under the clamshell.  There are three.  They are pull up valves, not with valves. To winterize, is this correct--?

Drain gray and black water.  Open three drain valves noted above and drain.  Close valves and fill water reservoir with 2 gallons of pink antifreeze.  Pour about 12 oz down each drain.  Turn water pump on and then open one water outlet at a time until antifreeze is visible.  Flush toilet.   Hook up low volume air compresser to water intake and open one water outlet at a time.  Reopen and drain three in line valves, then leave open over winter. Re-drain black and gray water reservoirs. Run about two gallons of bleach through system in Spring to sterilize water lines.  Then drain bleach.  Run city water through system and good to go?

Am I close, I hope?  Am I missing anything?   I really don't know what glycol is all about in the Alde, either.  Pretty mechanical but this is all new to me!

Thank you all!  This forum is so helpful!