Sirius/XM radio is somewhat of a confusion when it comes to 'Sirius/XM READY' vehicles - as it's only really an 'on screen' ready configuration, but you STILL have to buy your own 'radio' and install it within the rig - but it gives you the existing panel to control it by.
Now, some folks like me don't like that option, and just use a basic 'plug in' radio that simply plugs into the 'Aux' jack for your existing vehicle unit. I mounted mine on the dash nearby with two screws. I can also take it out anytime and use it any other of my vehicles.
As for 'memberships', nothing is required, and you may find that none really meet your criteria for how you 'camp', or 'travel'. Some will like the PassportAmerica 50% off deal, which is truly a great buy if you are willing to travel to those rv parks and campgrounds that accept it, with limitation by the park as to when and how they can be used.
The GoodSam is also fine if you simply want 10% off the 'normal' price at parks that accept it, but it also gives you a discounted price at CampingWorld retail stores, which I find to actually be much more useful than the park discount itself.
And, there are the campground 'memberships' that are similar to time shares, where you basically pay an upfront price for either free, or heavily discounted, stays, but only at 'their' parks, and possibly only at certain times of year, or with a future reservation, or only at certain 'sites' they allow.
None of these are going to always satisfy any traveler, especially if you travel extensively and during all times of the year, but a combination of several sometimes can be a better financial outcome that only a single membership.
two Campers, two Electric cars
former diesel pusher traveler