Remove and reinstall black tank?

I had a Mallard , the tank was set during the build of the trailer , the only way to remove it after the fact was cut out a frame support .

I liked what a previous poster said about the floor jack and bringing the tank down with the vent attached to it. Once down you can saw off the vent if you want the tank out from under the trailer and then dremel out what is left of the pipe in the tank. One thing to be aware of is that both your grey and balck tanks probably vent through the same line so they are joined somewhere; find that before you drop the tank so you know what you’re dealing with and if you have to disconnect that first.

Recruit help, two people will make this much easier!

I'm trying to expose the underside as much as possible cause I had water damage.

I've done it once on my Jayco. Now that I've done it once this is how I would try it if I had to do it again. I think I posted this once but it was years ago. If I find it I'll post again. But in essence;

Drain the tank. Use your leveling blocks to tilt the trailer to make sure the tank is empty.

Remove the toilet (unscrew the water line and the two bolts and lift.)

Pull the screws out holding the toilet flange to the floor.

Try to lift the toilet flange up. You might have to use hammer and punch to unscrew the flange off the top of the tank, I did.

Go up on the trailer roof and remove the screws and remove the vent cap. Make sure the vent line is free to slide down out of the roof as the tank lowers.

Under the trailer:

Pull any underbelly covering to expose the tank.

Take a digital picture of the level sensor wiring colors, then remove the wires but put the nuts back on the posts so you don't lose them.

Disconnect the drain piping and move it out of the way.

Get a small floor jack (the kind on wheels not a bottle jack) and a piece of wood (maybe 18 inches square)

Use the jack to hold up the tank with the piece of wood between the tank bottom and the jack (it's bulky, but not really that heavy)

Remove any screws that hold the tank up.

Slowly lower the tank a few inches and check that the vent pipe is coming down. If the pipe is coming down lower it a bit further and check again.

As soon as you can, slide your hand on top of the tank. Reach up and see if the vent pipe will pull out of the top of the tank. Mine was glued, but some are just slipped into a rubber grommet.

If the vent is glued in you can slowly lower the tank and then hacksaw through the vent pipe. Make sure you leave enough room to glue the pipe back together when reinstalling. On mine I just replaced the entire pipe as it was just a piece of 1 1/2 black ABS drain pipe. If you go this route you'll have the feed the new pipe in from the roof and down to the tank.

That's it in a nutshell. So what's wrong with your tank that you need to remove it?

I don't think there's one set of generic instructions for doing this. Every trailer is a bit different with drains and fastening schemes.

The short order of things is to cut or unfasten any drains and the vent. The tank should be supported by a strap or it can have individual fasteners on the perimeter underside. Remove the supporting hardware and any drain hangers. The tank should lower to the ground. I've made this sound easy but it's actually troublesome in most cases. Access to drains and the vent on top of the tank will be limited.

Why do you have to remove the tank? If it needs repair, most can be repaired in place.