Which AC filter to use?

While on our vacation in Florida last week, we returned to the camper one day to find water dripping from the roof mounted ac unit. I pulled the vent cover and filter, and found the drip pan was full and not draining. I cleaned the drains the next day and all was well. I have been using a filter designed for a house ac window unit. Since the filter was soaked, I ran the ac without a filter. The next day the filter was dry and I reinstalled it. My wife noticed how much less air flow we had with the window unit filter installed vs. no filter. I told her that no filter was not an option, it had to have one. Previous owner didnt use a filter and the evap core was completely plugged when we bought it. So what kind of filter does everyone use? I hope to find one with better air flow but still filters dirt.