Remember that the nerves that cause you to orgasm are NOT affected by RP - just the ones that cause erections. So for masturbation (and oral) you can still achieve very pleasurable orgasms without the erection.
For penetration you need a way to get the blood flowing into the penis and then to keep it there. Typically you start with a VED - vacuum erection device that you have probably seen advertised and available from sex shops (cheap), chemists and specialist medical suppliers (expensive) or possible on prescription if you are really lucky.
These are used with a 'cock ring' - a band that fits at the base of the penis and keeps the blood in the penis for up to 30 minutes. This can be used for penetration but some relearning of technique will be needed along with a good sense of humour.
There are also 2 injections that can help if you can't get on with the VED - I haven't tried these yet as the VED is working OK but will talk to my urologist when I see her in January. Maybe some others can advise.
Finally there is a penile implant - might be useful to consider if you are a 'show-er' rather than a 'grow-er' - there are two versions - a foldable one and an inflatable one but some people don't rate the very highly.