Short answer: you just can’t use on 7 for the Maxbotix using the libraries and sketch as you have it written.
Long answer: It’s a pin change interrupt conflict problem. The software serial library version you’re using is a modified version that only has access to vectors 1 and 2. Pin 7 is on vector 3. The ReadMe for ModularSensors has more details about how to deal with this type of conflict in serial input libraries. I might be able to give more details later if you need it, but that’s the ghist of it.