Most shower trucks and other shower facilities are open in the overnight towns the day you arrive. They are easy to find and probably near the campsite your charter has set up. As for in the morning you leave town I am not so sure. I am a morning shower person at home and have never taken a morning shower on RAGBRAI. I just get up and ride.
Breakfast can be found in the overnight town but most is found in the pass-through towns. The first 2 towns usually have many breakfast options. Most first timers have to experience Chris Cakes, which is always in a pass-through town at the beginning of the day.
Cash is good and how much you need depends on how much you eat and drink. Established restaurants take credit cards, but many vendors, church groups and other food sellers want cash. I allot $50.00 per day and that includes dinner. Some people need more, some need less, just depends on your intake. Many days I have left over, but I do not drink alcohol during the day. I do have a smoothie or 2 and they are $5.00-$6.00 each. Typical lunch food can be $5.00-$15.00.
Have a great first RAGBRAI.