I think what you will find at FW in January is a lot of retired people. They may have plenty of time, but I would NOT say they have plenty of money. My parents have an RV and they do NOT have plenty of money. They do, however, spend the month of January at FW. It is extremely economical for them. They have annual passes and spend most days at the parks. But, they eat plenty of meals in the camper.
One of the reasons the parks are not crowded is that you do not find as many day guests as during vacations and summers. The families are not there. We spent 11 days with my parents this year and we saw very few school-aged children. There were retired people and families with small children. It is a very nice time to go.