CT showed splenomegaly. Blood work had high RDW, low creatinine, and low MCH (anemia?); other CBC normal. How could anemia be related to spleen?

Splenomegaly. It isn't clear from what you say whether your hemoglobin itself is truly abnormal or whether just the RDW and MCH are altered. These in themselves do not define anemia. A large spleen may be related to trapping altered red cells, or actually making more blood (extramedullary hemopoiesis.) Often a high RDW means that there is increased production of immature red cells after destruction in spleen.