I had the same GI issues after my RP. More frequent (and loose) BMs and the very annoying (did I say very annoying?) condition called tenesmus (the feeling that you have to have a BM but there is nothing there). Some days were more normal, other days more intense but not so bad that it limited my normal activities.

Believe it or not I started a GI diary in spreadsheet that recorded times, symptoms (complete with a rating scale), foods, stressors, etc. in a futile attempt to figure out what were the triggering factors. So for months each day ended with "Dear Diary, today was a (good/bad) GI day...." No patterns emerged from my detailed multi-parametric correlation analysis, other than to stay away from certain foods (like beans and high/hard fiber veggies). No Nobel prize for this discovery.

A year later I had salvage radiation, which worsened the symptoms. My GI tract is still in recovery mode but improving very slowly (did I say very slowly?).