What the heck are these things for?

one end is larger than the other
it is supposed to aid in pulling the fumes from the tanks by creating a low pressure on the exit side (rear)

Agree with Movin'on.

I would guess the T is to allow a suction pf tank fumes while going down the road. I also wouldn't worry too much about rain getting in. You would have to have a rain of biblical proportions to cause a problem with water making it to the tanks.

Just take the T off and put a vent cap on.

Maybe 90' ells would be in order to avoid water invasion! An option to screening would be to put a short extension on the ells and a bolt through the pipe an inch or so from the end and then stuff the small space with stainless steel wool and a second through bolt to hold the steel wool in place. The finer steel wool would thwart more of the crawlies and would not disappear.

AHHH! so the Tees are "vent caps". now that makes sense.

They are not teed together ... there is a tee at the top of each vent pipe, rather than a vent cap. The tees are about 6 feet apart, but not connected.
All openings are clear at this point, but I will definitely cover them with screening.
Thanks to all,
John

I've never seen vent pipes Teed together on the roof before. Do you any pics?

J0,

Now that you know that, it would be smart to screen the open ends. Bugs love those.
BTDT

Matt

Now that you know what they are for, check them on occasion to make sure they're open and no creatures have built a nest inside.

Enjoy your Admiral.