You should be fine. The coefficient of thermal expansion for MDF is 12 micrometers/ meter - degree K (thanks google), so let's say you have an extreme day, where it starts at freezing and ends at 100F, that is a swing of about 40 Kelvins. Your table is about a meter long, so we are talking about a length change of 480 micrometers. That translates to a lengthening of less than half of a millimeter (0.48 mm). If the temp swing is that much every day, you might start to see some joint loosening. But I suspect that your dark storage unit is rarely if ever going to have that big of a diurnal temp change, more like 10K on the hottest summer days, so now we are looking at 0.12 mm. This is on the order of human-hair thickness. The glues and screws should handle that with no problem.
Your humidity is a non-issue :-)