Population map, from Wikipedia:
Modified map showing the number of nudist resorts in each state, based on the AANR club listings:
Analyzing the data in the above graphics, it's reasonable to conclude that the following states have a large population and a shortage of nudist resorts (compared to similar states):
However, since this series is about urban resorts, one must also consider the population density maps.
Population density map from Wikipedia:
Many of the states which contain no nudist resorts have a low population. But, the majority of the population is within cities. States which have no resorts and have a high urban population density include:
If I were to create an urban nudist oasis, I'd put it in one of the states above.
Many of the states happen to be in northern latitudes as well. Nudist resorts, for the most part, close down during the winter months. Most only have outdoor facilities. Bigger ones have a few indoor facilities, but it's often not worth venturing down slippery backroads just to swim in the nude. An advantage of an indoor urban resort is that it can remain open year-round and provide the same facilities.