OK problem is, my dad in-law has a mobile home and neither of the toilets will flush out. He recently bought it and had it moved hence this is when the problem started. The toilets have not flushed out since day 1. Everything else seems to run out OK, shower sinks etc... Could this be a venting problem? Laymen terms as I am new to plumbing haha. I learn as I go from AMHD!!

I would suspect the installation of the drain at the new site. Double check that first. If you suspect a clogged vent just rod from the roof, run in about 15' of snake.

Only thing is I do not see anything vented through the roof of his mobile home. Could it be vented in other ways in this trailer? I looked at the drain, both of them. They seem to be in the correct positions. They bathrooms are at either end of the MH yet neither toilet will flush out all the way.

They look to follow the rule of 1/8 - 1/4 inch fall for every 1 ft pipe etc... Kinda got me stumped. I can't see why they won't flush out.:confused::confused:

I don't work on mobile homes very often but one of the plumbers used. I'll ask him to stop by.

Ok sounds good.

I live in a newer triple wide and my vents are in the walls seems like and run straight out through the roof.

But with his roof I see nothing.

Back in the 70's I worked in mobile home factories as a plumber. In all of the places I worked and in every trailer I've plumbed the vents were terminated through the roof.
Now that was years ago but plumbing basics haven't changed over the years.
I know that in some of the plants that a few units slipped by with the vents still inside the walls and not exited through the roof. Now this produced a sewer gas odor that I don't se a complaint about from 500. Since mobile homes aren't inspected by a Plumbing Inspector that no vents were installed at all. On your father in laws unit.
You ask, "

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I do not see anything vented through the roof of his mobile home. Could it be vented in other ways in this trailer?

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It's possible but not very probable. The manufactures like to cut costs wherever possible. It would cost more to vent with AAV's (Air Admittance Vent) then it would to run the vents out the roof.
My advice would be to get a plumber in to check and locate the vents. At the same time he could check on that "lazy flush". Good luck, Tom