You answered your own question.

High temp.
Full sun.
BLACK nursery pot.

My potted trees and bushes get watered every other day here in south Florida by necessity. If you shade the pot and mulch the top of the soil you will lose less moisture.

Gary

Thanks newgen! What can I do about the full sun part? Thats all my backyard has besides a full shade porch!
gnappi- I have mulch, how do you shade the pot?

Man-Go: My yard is south facing, full sun, dry heat, high temp. For containers, I've seen the following done:

you can double pot: place the pot with the plant inside a larger pot and fill in the space between the 2 with bark.

Or you can wrap some light-color material around the pot, to reflect the heat.

Or you can build a shade structure for your pots, depending on how many plants you have in container. Just 4 posts and some shade cloth overhead. What about moving your 2 containers under the shade porch?

Can you plant your trees in the ground? Any reason why they have to be in containers?

The papaya is in the ground, and reasons why my mango isn't are:
-Danger of freeze damage (ability to be moved indoors)
-Lack of space left (I already have 9 fruit trees in the ground and a lot of other junk that needs to go)

I have my potted trees in full sun all day too.
It sounds like you need to water more frequently.
I fully saturate until the water runs out from the bottom every other to every third day during the hottest months of the year depending on the amount of rain we get. You could get one of those fold and pack picnic canopies to give the trees some shade mid day. Mango trees are by design capable of handling full scorching sun all day long with proper hydration so I would start with ramping up the irrigation as the first step and then reassess the outcome. Happy Growing :)

Thanks marinfla!
Does anyone think I could successfully grow a mango tree in the ground here in TX? (z9a/b)

I have a NDM and it is in its 3rd summer this year. Our summer here is in the 100F or closer to it every day. 1st year it was in a black nursery pot. The tree did not during the entire summer except some growth in spring and fall. Last year I moved it into a 17" clay pot. I hoped this would helped cool the root zone. But it had done no difference than the year before. So last week I dug a hole and buried that mango tree with pot in ground and mulch on top. Will see if there is any difference this summer.