You shower before you clear customs at FRA. The RCC is inside customs. Also, there are 2 or 3 places to clear customs at so if one has a big line, you can go to the other.

Interestingly, I flew on UA 900 LAX-SFO-FRA this week. The LAX-SFO portion was in F, the SFO-LAX was in C. When I went to the RCC I grabbed my boarding pass out to show them and it was the LAX-SFO portion so they directed me to the F lounge.