In Tx we've found the cheapest parks are the Corp of Engineer (COE) parks (federal parks) that are administrated thru www.recreation.gov. With a Senior Pass for National Parks (you're eligible to get one if over 62) you'll get 50% off the normal rate which is already pretty cheap. We normally pay $12-15/nite. Your limited to 14 days at a time, but you may be able to find another COE park in the same area & just bounce back & forth between the 2 for however long you wish to stay in the area. Waco, Tx. is an example of an area that has 2 or 3 COE parks in the area.
Tx. State parks are pretty cheap also. There's a Tx State Parks Pass you get for a year at a time. Can't remember the cost, but somewhere around $75. It allows you access to the parks for free, but you still have to pay the normal daily rate for a camping site. We like the COE parks better cause they're not only cheaper, but you can reserve a specific site in them whereas in the State Parks you just reserve a site in general based on size of your rig then get what's available when you show up. I think you're also limited to 14 day stays at the state parks, but not sure.