I have almost given up on park WiFi services. As was mentioned they are most often hit and miss and can get very slow when the park is full in the afternoon/evening. For the past 3 year I've been simply tethering my 4G Verizon phone without any problems.
We travel for 4-5 month through the western stated and have yet to find a hole in coverage. I even leave it on while traveling so DW can use her tablet. Like others I'm still grandfathered in to the old unlimited plan and use about 20 GB of data per month.