Tadalafil can be taken at a low daily dose (5mg) to promote blood flow with the hope of repairing nerves after RP. It can be taken as a one off 'event' dose which would be either 10 or 20 mg. If I take 20mg I will have spontaneous erections for about three days afterwards that is great, but I also have three days of acid reflux.
If I were in your position I would go on an online pharmacy and get a selection of ED medication and lanzaprazole or over the counter omazaprol and do some experiments. Remember medications can have interactions and you may have other medical conditions, so make sure you are aware of all the risks you are taking.
I am a well intentioned random person on the internet and no substitute for a professional medic.