The filter will filter out places that are not marked as big rig friendly. However, this is user supplied data. If a user said the place is not big rig friendly it may be just because they were not comfortable driving their rig in the campground that does not necessarily mean you won't fit. The other issue is that "big rig" means different things to different people. There is no enforced standard (larger than 40 foot) that I am aware of.
It comes down to where you like to stay. If you prefer the concrete parking lots of rv resorts then you are pretty safe in assuming they can accommodate you. We like to be off the beaten path and in the woods if possible so we stay at a lot of state parks, country parks, and COE campgrounds. For those I have to go to the registration and filter available sites by size to see if they have any that will accommodate my 40 foot rig. If they do, I consider it big rig friendly enough.
TLDR: Turn that filter off you are missing some fantastic places to stay.