It's simple.

1. Put about a quarter cup of bleach OR liquid Lysol in the tank for half an hour. Do NOT MIX ANYTHING WITH BLEACH OR DEATH MAY RESULT!!!

2. Turn OFF FILL VALVE. Flush

3. Fill small bucket with the same solution and recommended amount of water for typical cleaning.....dilute.

4. Wipe with rag.

5. Turn fill valve on.

The time I took to write this your problem will be solved.
None of this will harm the seals, however you can't leave bleach in the tank for long periods.