[VAC] Fresh/grey water tanks/plumbing

["Robert / Susan Harper" <rharper@xxxxxxxxxx>]

    I checked the measurements of the space under the floor at the front of
the '62 Globe Trotter regarding a potential gray water tank location.  This
would be roughly in the position of the dinette table I have planned (but
below the floor).

Starting info:

- A porta-potty has replaced the flush toilet, so there will be no black
water tank (who came up with these names?).
- the original fresh water tank, about 10" diam x 56" long holds about 19
gallons (231 cu. in. / gallon)
- I can buy a new 18 gallon rectangular ABS tank (8 x 16 x 36) for about
$85. (We'll put the 19th gallon in the fridge since I moved up to 3.7 cu.
ft.)
- my new water heater holds 6 gallons
- there's a good chance that we'll have the opportunity to dump our gray
water before the tank fills up

That's a potential maximum of somewhere around 24 gallons of gray water (or
at 231 cu.in. per gal,  around 5544 cubic inches), and that assumes
completely emptying the fresh water tank and the water heater tank. That's
probably not likely, since the last 6 gallons would be hot water, if it
would come out at all (will a Shurflo pump empty out a water heater?).

Without moving the existing floor stiffening steel angles (run front to
back), which are simply bolted under the plywood floor at the front (not
welded to the frame at the ends as are most of them on my trailer), I come
up with a rectangular area measuring 26" x 45", or 1170 sq..in.  If a tank
measured 5" x 26" x 45", it would be 5850 cubic inches, or a little over 25
gallons. I think that's do-able in that space. If I put 3/4" of rigid foam
insulation on the plywood first (to keep the toes warm - thanks Terry), then
a 5" custom ABS tank, the bottom of the tank would be 5 3/4" below the
bottom of the floor, or 1 3/4" below the main trailer frame members. If it's
1/4" wall tank (5" inside) then add 1/2", for a total of 2 1/4" of drop.
That's not as low as my hitch, my jack, nor my axle, and that "compartment"
would be pretty easy to seal with a separate section of belly pan.

But of course this brings about some questions and potential problems. My
intention with the shower at the rear bath is to use a slightly contoured
stainless steel floor pan with a depth of no more than 2". The drain would
be at floor level or possibly 1" above, near the center of the trailer,
about 2' from the back.  That's a LONG minimally sloping drain line to run
all the way to the front gray water tank. And if the gray water tank in
front is half full and I pull this trailer up a steep grade, am I going to
open the door and find 15 gallons (or more)of gray water has backfed through
the shower floor pan (and out into the rest of the trailer)? This would not
be a good thing. I know a trap won't stop that, and the backflow prevention
devices I've used in irrigation might need more pressure than draining
shower water to open in the flow direction.  If I put in a shutoff valve,
we'll forget to open it or to close it.  Closet plumbers: help!

I could put an additional small gray water tank at the back for the shower.
I could use one of those portable tanks for the shower. I just hate making
extra work ... extra things to remember to do each time we use the trailer.

Suggestions, experiences, ideas?  (The pipe someone welded to the back of my
bumper would hold 4 1/2 gallons ..... )

BH62GT/SC