I would more clearly define "not super high" - expensive is relative. In Florida we think anything below $600 per month in the winter is "cheap"...
not for seniors but we like Catherine's Landing RV park in Hot Springs AR. Pool and access to Lake Catherine and plenty to do in the area. Lots of retirees in Hot Springs. Their current back in monthly rate is $545.99.
Another of our favorite parks is Spring Lake RV Resort in Crossville TN. It's on the Cumberland Plateau and is cooler than the valley. No pool and not many activities except small fishing pond (catch and release). It is a very quiet and relaxing place. The town of Crossville is very friendly.
also Sante Fe NM and north NM seemed cooler when we were there.