4. Roundup residue still in soil? I have never used Roundup (and never
will) so I am just guessing on this one.

Roundup doesn't persist like some herbicides do, so this one isn't
so likely (although we don't know what the time lag was between the
roundup and the tomato planting).

As for the 6 hours of sun, that will affect fruiting more than growth.
And actually, if those are morning hours, that might be the way to go
in such a hot, dry climate. (We're not in New Jersey anymore....).
If this is the problem, picking an appropriate variety (cherry
tomatoes, probably) is likely to help.

Agree about overfertilizing. Lots of compost could even be an issue
(if the compost is fresh), although that one will resolve itself for
next year.

As for the nighttime temperatures being too hot, as I understand it
the reason people mention that for tomatoes is in terms of fruit
setting. So this probably is indeed a problem, but the lack of growth
is probably more a question of daytime temperatures, the really dry
air, or some different cause.

The black plastic is probably making the soil too hot. Maybe try
white plastic or something else? (I'm not really sure what's best).

As for Desert gardening in general, the one place which very much
sticks in my mind is the Forestiere Underground Gardens in Fresno, CA:
http://www.roadtripamerica.com/places/forest.htm

It is really amazing (lush and green and sunny, despite being
underground). Now, I'm not sure how much of that can easily be
applied to a home setting, but think in terms of things like
shadecloth, mulch, anything you can do to keep the plant's environment
from acting like an oven. That hot sun beating down is the big issue
in this climate, and irrigation is only part of the answer (for one
thing, constant irrigation leads to shallow roots).