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Old 12-10-2019, 12:29 AM   #1
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Turning black tank into fresh waster tank...

I was wondering if anyone has purchased a NEW (OBVIOUSLY) Class C and connected the never used black tank to the fresh water tank in order to double (or more) the freshwater capacity?
I ask because I fully intend to buy a new Class C and replace the standard toilet with a simple composting unit. Therefore the black tank would be available to hold extra fresh water. A few simple pipe reroutes should make this possible. Has anyone done this before?
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Old 12-10-2019, 12:53 AM   #2
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I've purchased three "New" RV's over the years. On all three the black tank had been used.

So I suggest doing a sniff test on the black tank vent pipe on the roof before purchasing.
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Old 12-10-2019, 04:03 AM   #3
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The original black water tank may not be made from a potable water plastic.
If you're serious about this, the yank that tank out and install one you know is good and clean.
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Old 12-10-2019, 04:05 AM   #4
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Originally Posted by Arch Hoagland View Post
I've purchased three "New" RV's over the years. On all three the black tank had been used.

So I suggest doing a sniff test on the black tank vent pipe on the roof before purchasing.
OMG, that is ridiculous. The toilet should not be used on a new RV! Thatís so obvious!
I will DEFINITELY make sure it has never been used, lol... Seriously.

Thanks for the reply.
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Old 12-10-2019, 04:58 AM   #5
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OMG, that is ridiculous. The toilet should not be used on a new RV! Thatís so obvious!
I will DEFINITELY make sure it has never been used, lol... Seriously.

Thanks for the reply.
RVs are not trucked to dealers like cars.

If its driven from the factory, to the dealership, any distance, there is a good chance the driver will use it.

Unless you pick it up, right out the door of the factory, treat it like its been used.

There will be a lot of changes to convert it. You will need to isolate the discharge pipe from the gray, check that the vent stack isn't shared and figure out how to fill and draw water from it.
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Old 12-10-2019, 08:15 AM   #6
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You will be better off using it as a second gray tank. You wonít have to do any modifications other than installing a third valve on your dump line at the point where the slinky hooks up. Open the gray and the black tank valves and allow the two tanks to equalize.
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Old 12-10-2019, 10:18 AM   #7
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Modifying an existing rig that way is likely too evasive. If it is something you want bad enough, consider working with a small RV manufacture like Phoenix USA who does custom work upon request all the time. Have them engineer it right for you.

The easiest and most affordable approach would be to add a second fresh water tank inside and let the black and gray tanks share the sink and shower waste water.
Our Phoenix Cruiser SEEN HERE, I can blend the waste tanks at the flip of a switch so waste water is shared between the black and gray tanks. The two tanks become one at that point. Because our gray tank always fills up first, when I dump, I first let the gray water backwash into the black tank, then close off the gray tank. I then eject the black water via the integrated macerator. This flushes the black tank with soapy water. Then I dump the gray.

With your setup with a composting toilet, I would open both gray and black tank valves and leave them open all the time as a combined gray water tank.

The only serious problem doubling your fresh and waste water capacities, is the extra weight, so be very mindful of that. My waste tank cleaning process of bleaching both waste tanks at end-of-season, I have both tanks filled to capacity with a bleach solution, soaking for a few hours before draining and rinsing them clean. The rear end of the rig sags with both tanks full. I would never drive the rig that way.
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Old 12-10-2019, 11:08 AM   #8
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Remember that water weighs 8.33 #/gallon, so 30 gallons will add 249.9# to the rig plus the other water. Can it handle the added weight.

I doubt if the original black tank material is suited to potable water use. Replace the tank with a new potable water rated tank.

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Old 12-10-2019, 11:13 AM   #9
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Originally Posted by VocalVirgo View Post
I was wondering if anyone has purchased a NEW (OBVIOUSLY) Class C and connected the never used black tank to the fresh water tank in order to double (or more) the freshwater capacity?
I ask because I fully intend to buy a new Class C and replace the standard toilet with a simple composting unit. Therefore the black tank would be available to hold extra fresh water. A few simple pipe reroutes should make this possible. Has anyone done this before?
Seems like others answered....but I would not change the black tank....resale...black tank is not potable water safe.
Just pack extra water...1 gallon jugs from dollar store. I put them I'm my road.
Or get a good water filter for boondocking.
Just seems like too many reasons not to do the mod and many ways to bring more water..
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Old 12-10-2019, 11:15 AM   #10
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OMG, that is ridiculous. The toilet should not be used on a new RV! That’s so obvious!
I will DEFINITELY make sure it has never been used, lol... Seriously.

Thanks for the reply.
Some MH get driven far and the drivers have been known to use the toilet. Aldo when going to rv shows. Dont know if you have been at rv dealers when a show is happening. they hire what looks like anyone who can drive a MH. Basically hired drivers. It happens.
Unless you get direct. The toilet could have been used but by no means is it anything bad.
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Old 12-10-2019, 11:29 AM   #11
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The toilet could have been used but by no means is it anything bad.
I would not want to be the one to find this out the wrong way by getting sick!! This whole topic spells wrong on so many levels that I can't even begin to comprehend why you'd even want to do this with a black tank? Pull the tank and use a fresh/correct type of tank if your so set on doing this. Your health and safety will love you for it! Good Luck
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Old 12-10-2019, 11:37 AM   #12
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I would not want to be the one to find this out the wrong way by getting sick!! This whole topic spells wrong on so many levels that I can't even begin to comprehend why you'd even want to do this with a black tank? Pull the tank and use a fresh/correct type of tank if your so set on doing this. Your health and safety will love you for it! Good Luck
You cherry picked a single line.
Was referring to the toilet being used and sold as new....please re read my post

My post above the one you quoted states it would be unsafe to use as potable water tank.
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Old 12-10-2019, 05:26 PM   #13
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Some motor homes have the bathroom sink dump into the black tank as well as the toilet. Make sure you know how the coach is plumbed.
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Old 12-10-2019, 05:54 PM   #14
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Some motor homes have the bathroom sink dump into the black tank as well as the toilet. Make sure you know how the coach is plumbed.
You reminded us of that very good point.
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