So, I after I get the RTD, do I need visa to visit Germany?You can use either a US Refugee Travel Document or a US Re-entry Permit. Either of these is a passport-like booklet that many countries will issue visas for and accept for international travel.
Since you got US permanent residency through asylum, you can get a Refugee Travel Document. It is valid for 1 year.
Any US permanent resident (not necessarily through refugee or asylum) can get a Re-entry Permit. It is valid for 2 years.