I had no response to all big 3 oral drugs after RP.
I started trimix and got good results (and an ER visit for priapism). After some months of trimix, in ever decreasing doses, I found that oral drugs were starting to work.
I now cut a 20 mG Levitra in 4 and that seems to be more than enough...and often without any meds, too.
In my case, I believe trimix gave me a bit of a jump start. Hope it does for you, too.
I still get the congestion and flushing in varying degrees. But, it's better than it was.
Good luck.
Mike