I guess it depends on several variables. I would think that a reasonable window unit would cool, eventually, unless it's 100 degrees out in direct sunlight. In our 28 class A the roof AC will cool it down nice. If we're at the beach in the direct sunlight on a hot day it will stay cool in the RV. But, it's 15,000 BTUs. On a hot day I would doubt that a 5,000BTU AC could keep up. In the grooming van we have a nearly new 13,500BTU and on a hot day with the washing and running the dryer it will really struggle to keep us cool in a 19 foot van.