5000 BTUs is not a lot, but should be okay in a small space. Ambient temperature makes a huge difference, along with relative humidity and direct sunlight. I have to draw all shades in the height of the summer to get decent cooling and I have a total of 42000 BTUs.
How does the portable unit exhaust it’s own waste air? I assume these are air cooled, not raw water cooled.
Can you close cabin doors and limit the space it’s cooling?
You might also buy an inexpensive digital thermometer gun ($10-$20 on Amazon) and check the actual temp of the cooling air at the unit. You’ll have to look up what it’s supoosed to be, but perhaps it’s underperforming. Is it new or used?