At one point there was the requirement that all apps do something and that an app can't require a user to enter personal information. The solution was to have the app do something - in your case perhaps a map of truck stops. Then if the user wanted to they could enter their login information and get extra functionality. I'm not certain that requirement is still applicable (e.g. Uber).
One issue is a 'trust me' issue. You need to convince App Review that you are not actually selling the functionality using a means other than IAP. You aren't - but you may need to convince the App Store that the custom login info isn't really a ruse to charge $9.99 (and get 100% not 70%) for use of the app?