Several factors play into what is causing the mold growth. First off
unless you have a leak issue covering the pop up in storage only helps
with trapping the moisture, secondly I would recommend drying all parts
of the pop up before closing and then opening it back up when home. Mold
is difficult to eliminate, not cleaning but eliminating what is causing
the growth.
I manage a very large contact account for several business' and
encountering mold is an up hill battle, the first thing we look at is
the air conditioning unit. If the A/C unit does not drain away water and
moisture this becomes the triggering mechanism for the mold to start it
growing within other areas that maybe wet.
I suggest getting the cleaner as stated in the other post, but before
doing any cleaning check you A/C unit for proper draining, check for
potential leaks  and then remove all interior materials including
mattress', drapes, capet throw rugs etc. If you have several good days
of dryer weather or an area that is low in humidity I would keep the pop
up open in these conditions to "air kill the mold spores". Please be
careful using bleach, cleaning with the product is very safe and
suggested but do not pour bleach into areas that will allow it to stand
for periods of time and mixing it with other chemicals can be dangerous.

I hope this helped.

Rip