Your problem isn't humidity. It's water, and temperature.
A closed oven which has a significant amount of food in it will be quite humid, in the sense that the gas inside it contains a high percentage of gaseous water (aka steam). It's possible for food to crisp up, and even burn, even with that steam being present.
But that high percentage doesn't equate to much mass of water. When the oven is hot, the density of the gas is low.
Assuming you preheated the oven, it was hot. But when you open the door and see a big cloud of vapor, that means that the oven had cooled off, probably because of evaporative cooling (as water evaporates off the food, it cools both the food and the air). It's the cold oven that's limiting your crisping.
If there's a lot of water, and a lot of surface area for it to evaporate off of, and not enough oven power to build up a temperature gradient, the food can effectively hold its own temperature down. This is what causes "the stall" when BBQing, and it's what causes limp, moist oven fries.
Potential solutions:
- Increase heat flow to the food by using convection if available, or by using all heating elements if your oven has multiple independent elements.
- Decrease evaporative cooling by cooking less food at once.
- Cook longer. Once you've dried out the surface of your food, evaporative cooling stops, temperature rises, and crisping/browning picks up.
- If you weren't preheating your oven, preheat your oven. The oven walls have a high heat capacity which buffers the evaporative cooling.
Things that are not potential solutions:
- Opening the oven door. I know it seems like that's "letting the steam out", but the steam was going to come out anyway; you're just letting the heat out with it and slowing things down.
- Increasing the temperature. This doesn't make your oven work harder! When it's being limited by evaporative cooling it's already working as hard as it can. Redlining the thermostat is just going to put your food at risk of burning.