Although it varies somewhat, bleach in the jug has a concentration of about 50,000 ppm. We all know what spilling straight bleach on hands/fabric can do. Hence the fears about using it. The amount in "city water" is usually close to 0 (it gets used up on the way to your home). In a pool, it can be from 2-5 ppm. Without protective other chemicals, in a pool it can break down in less than an hour, due to sunlight. The end result in pools is table salt (NaCl) and CO2 after breaking down. If a lot is used up killing things, a by product is chloramines, which produce that well known "pool smell" - so the stronger the smell, the dirtier the source water. Clean water with that low of a Cl concentration has almost no smell.
In RV's, the diluted concentration using the typical "recipes" is 50 ppm. While far below recommended levels for rapid sanitizing (spraying on things, brief soaks, etc.), which is up to 5000 ppm, the added factor is time - let it work for hours rather than minutes. Without sunlight, it can persist longer to do its job.
Tanks/pipes/hoses grow things if there is any moisture at all. Always. It is generally harmless bacteria or molds (except for those with sensitivities). They may make the water smelly or taste off. Only if you start with untreated water is there a risk of things that will make you sick. (Streams, lakes, untested wells, etc.) Because city water has no to very low concentrations of chlorine, it will do almost nothing to combat anything growing. But generally, those that don't sanitize aren't taking much of a risk, either. At worst, the water may seem a little "funky".
That 50 ppm amount breaks down to some level doing it's job in the fresh tank and plumbing. When it hits the gray tanks, where there may be LOTS of things growing and organic residue (soaps, food, shower residue, etc.) it breaks down very fast. It can be all over the map as to how much is left over when you dump.
Some places regulate how to dispose of chlorinated water (like emptying a pool). The concern is the huge quantity (5,000-40,000+ gallons) that may make it via a storm drain to a nearby stream/pond/lake and use up its chlorine disrupting the local biome. Hence using a sewer to a local treatment facility is preferred.
But, the amount of water we use in sanitizing our RV's is a huge amount less, and therefore a huge amount less of chlorine being disposed of.
When dumping 50 gal in a yard? First it is not a lot of water, second the concentration is already less than one started with, third most gets used up almost instantaneously on contact with the soil organics, fourth any exposed to sunlight is broken down VERY quickly. With all of that, the impact to local vegetation is virtually zero and there is nothing that persists to migrate to wells or ground water.
Going to a dump station is the greatly preferred method. But very occasional ground dumping has no short or long term impact to be feared. Other than plant damage (due to the very high concentration) would there be any other issue if you spilled that original 1/2-1 cup of bleach on the ground?
BTW - pool test strips are notoriously very inaccurate, and generally can't read concentrations higher than about 10 ppm. So other than a guess that "there's not much left" they are pretty ineffective.
And - to those that struggle to get rid of the "bleach smell" after sanitizing...well, that smell is a flag that there was a lot that needed to be sanitized in the system.....
Me? I worry more about what other smelly junk may get flushed out of the gray tank, onto my lawn than the left over bleach water.
