There are ways to rig the valves to make the gray water go back into the
black water, but there are much simpler things to do that don't involve
fooling with the hardware. Here's a few:

Use a dishpan in the kitchen sink and empty it into the toilet.

Don't leave the water running when doing dishes. Wash them in the
dishpan then rinse them with the water turned on 1/2 of full force.

Use a plastic bowl(reserved for this purpose) in the lavatory and empty
it in the toilet.

Take "military" showers. Wet, shut the water off, lather, turn the water
on and rinse.

Don't leave the water running when shaving or brushing your teeth.

The conservation of fresh water use is the best defense against full
gray tanks. JMHO

Hi,
It's usually the other way around with the tanks most gray water tanks
are equal to or bigger then the black water tank.  I have a 70 gallon
gray tank and a 40 gallon black tank, even then the gray water tank
always fills up first.  Anyway the only thing you can easily do is
conserve water by not letting the water run unnecessarily when taking
showers, shaving, washing dishes etc.  Or by using dish pans etc. to
hold the water when washing dishes then dumping it down the toilet.
Retro fitting the plumbing is not an easy job on most RV's.

Take care and Happy Campin...

Great question as I also have a larger black then gray. I do the consrver
water thing such as wet,wash and rinse but sometimes my guests are not aware
of the issue.

One solution I found is a portable gray water tank that is towable (has
wheels and a hitch handle). (About $100) just make sure the one you purchase
fits in your storage compartment. I almost made that mistake.
I keep an extra 1 1/4 and 2" hitch bar and ball in the RV so that if one of
my friends has a hitch on the back of there car I can tow it without moving
the RV.

So for about $150 you can have an extra tank.

Ed

Drain the gray water on the ground.

One day prior to leaving a primitive campsite, I close the valve to
capture/store enough gray water to "rinse" the dump hose after emptying the
black water tank at the dump station.

Purists will have a field day with this practice but I don't care.  I never
captured gray water when tenting - it went on the ground.  There is no
difference just because I now use an RV.

Of course, in those areas where draining gray water on the ground is
prohibited (i.e. federal campgrounds), I use a 30-gallon Tote-Along tank.

http://www.barkermfg.com/product_pages/tote_alongs.html

When the on-board gray water tank is full, I drain-off 30-gallons and haul it
off to the dump station.

Never mix grey and black water. Urine reacts with soap to produce
formaldehyde gas.

That is why the two tanks are separate and not just one.

NM