Originally Posted by -cycosis-
I would love it if you had some pics of your setup? This is where I am most confused about my project...
Sorry, no photos! I'm photographically challenged (and that's being kind).
Even though I thought that my tanks' installation was a difficult project, looking back on it now it actually was quite simple, at least compared with some other things I've done. You first have to find space for the tanks you have, or buy tanks for the space you have. I suggest having the most possible water capacity - just because you have X gallons of tanks doesn't mean you have to have them 100% full all the time, but there may be occasions when having lots of water is good. Make sure the tanks cannot move at all in any of the six possible directions of movement (up, down, left, right, forward, aft) - this means positive location by using something solid and strong to keep them in place. As I mentioned, you can control their horizontal movement by cradling their tops within a surround frame under the floor, then as long as they are held tight up against the floor they cannot move at all. It's simpler and better to do it this way instead of making diagonal or cantilevered bracing against their bottoms.
You can drill holes through the frame rails' lower flanges, but you have to drill them along the flanges' center line where the stresses are neutral. The tanks should be supported underneath all around their periphery, and underneath every foot or two to prevent them bulgeing when full. Polyethylene tanks should be protected from debris by being sheathed in plywood. If you have roto-molded tanks, it's worth having all their fittings spin-welded in, and use the thickest heaviest-duty fittings available to lessen them stretching as you tighten pipe fittings into them.
Fresh water tanks need vents to equalize pressure inside as they fill or are drained - a 1/2" line should be plenty. Black and grey tanks however need much more venting. You need to allow fresh air in to let the aerobic bacteria do their thing, you need to let any methane harmlessly out, and you need to let lots of air back in when dumping their contents. Most folk use a single 2" or 3" vent up through the roof. I did something different. Each of my waste tanks has two 2" vents in diagonally opposite corners of their tops - the black tank's rear vent goes up through the floor and behind the closet, and exits through a louvered marine hose vent high up on the bus's side just under the roof gutter, with the louvers pointing forward. The black and grey tanks' front vents are interconnected, and the grey tank's rear vent goes down to ground level and points back with a large opening just ahead of the differential. When I'm driving, air is forced into the black tank through the forward-facing upper louver vent, flows through the black tank and into the grey tank, and is pulled out through the grey rear vent by venturi effect as air rushes under the bus. This way I have positive airflow through both tanks while driving, and any smells are exhausted at ground level so they cannot be sucked back inside. When parked, natural convection causes cooler air to enter through the ground-level grey vent, through both tanks (the opposite direction than when driving), and up through the black's upper vent, aided by that vent pipe being next to a panel that gets warm in the sun so the air in it heats up and wants to rise, pulling the air in the tanks behind it. It sounds more complicated than it is, but it seems to work just fine.
And you thought tanks were simple!