We were never impressed with RPI. YMMV
AFAIK TT has a 'no turn away' policy everywhere except Vegas. If you show up (with membership) at a TT park without reservation, they will find you a spot. It may be without service, but you'll have somewhere to sleep.
We always try to make use of the reservation system. Understand that with TT you reserve space, but when you get to the park, its on you to find a site that fits your rig, has the services you want/need, has sat reception, etc.
To make this work in your favour, you learn to arrive around 'checkout time', usually 11 am or noon, and start your search for site as others leave.
If you do end up in a site you dont really like, take a walk around the park and look for rigs that are leaving in the next day or so (out dates are printed on the paper you display on your rig while in the park) in a site you want and knock on the door. Some cookies go a long way in getting a 'we are leaving in the next half hour' phone call the next day. Always move first and tell mgmt after! (Except for Pio Pico, (San Diego) which has special rules, thats another story)
You learn to work the system, rather than having the system work you.
Its all part of the adventure.
-Steve