We've used a collapsible/foldable water carrier with a water filler spout on grey water catchment set-up for our `88 Westfalia's sink drain for 33+ years, & for our 1970 Eriba Puck since we got it in 2018 (with drain size angle adapter), and the same would work for your Casita
While GoWesty doesn't list the collapsible containers right now, they're readily available at all camping outlets, online (Amazon as noted above), Walmart, etc.
Water Filler Spout for insertion into the container - clearly mark as "Grey Water" if you use them to also fill your fresh water tank:
90 degree Drain Adapter if needed for your set-up:
Another example of a collapsible container:
The above adapters & fillers are probably also available elsewhere wherever camper & camping supplies are sold
We've recently upgraded our `88 Westy with the Euro supplied Grey water holding tank, but we'll still carry the collapsible container etc. for easier draining of that tank if needed where a drain/RV Dump isn't available. The Euro Westfalia/"Joker" Camper tanks hold about 10 gals - as do the built-in Fresh Water Tanks on all Westfalia Campers of that 1980-92 Vanagon era (see attached pics below).
Unfortunately, there isn't a built-in grey tank made for our 1970 Puck due to it's mission to stay light
for car towing at only 660 lbs empty/dry & max. 1000 lbs GTWR - so we'll continue to use a drain hose &/or collapsible container set-up with it.
may have an aftermarket or factory optional grey water tank that you could add under or in yours, as another option - similar to the one on our Westy.
The above collapsible set-up should work fine for you - plus many campgrounds are getting smart again about using reclaimed/grey water for irrigation again in many places due to expanding droughts (it used to be widely allowed, until folks let the food crud run out too).
So you may find many will let you run your garden hose type drain hose into a planter to help water their plants - but make sure to use an in-sink drain strainer to keep the crud out of the discharge.