I have a reimo 11 gal mounted between the frame rails which I use for fresh
water.  But, I like the idea of using 4" PVC tubing cut to the proper length
and then mounted between the frame rails.  They are lower profile and would
be tucked more out of the way.  You could fit 3 across if you wanted lots of
volume, or just the one for waste water.  Since it's PVC all you have to do
is use PVC glue to tap in whatever fill/empty connections you want.  And
they also sell the caps for them to seal up each end neatly.

Dave
Syncro custom camper 2.5

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