I got the thing with the stars working. In one of the other threads about real navigation someone posted his NYGM version of the camera.dat. Now it seems to work allright for me.
My little patrol report using RealNav the first time so far:
Got out of St.Nazaire at Dec 16th 1940. Patrol area was ordered to DT59, West of Northern Africa. So I set course to 245°, 10.5kn.
Weather was fine till I reached the first waypoint 980km somewhere near the spanish coast. There we got into heavy fog and rain which kept going on for the next 10 days.
Course led me further southeast, had to go around Madeira, I took a safety course some kilometers further to the east then initially planned to not run into the ground because of the bad visibility.
Once I used Crtl + LMouse just to check my position, I will forbid me that on my next patrol

Course was good, I just overestimated my speed a little bit, so I hang a little bit behind. Perhaps this is a issue of TC, I used 256x or 512x at max.
When I finally reached the patrol area I made another Ctrl+LMouse check, the same about the speed again, otherwise our position was suprisingly exact.
On our way I cut down the TC every ~4-8 hours to correct the course again which slightly slipped off track from time to time.
I decided not to reintegrate the Nav officer feature that he tells me how far I can got with my remaining fuel. I just count the travelled km's and add them up.
So far in this patrol I travelled ~4141,1km. I have to take into account that stormy weather and recharging batteries will increase the fuel consume.
When I thought our 24h patrol in DT59 was over I had to CTRL+LMouse again because it said I didn't patrol 24 hours in that sector yet. I saw that I got a little bit out of the DT59 so I headed back south, finished the patrol.
Again it was a matter of higher speed than I though although I already took even half or quarter knots into account when making my calculations.
Suddenly in this night the sky cleared completely up again wind turned on to 13km/h though.
I did a reading of the stars and Polaris' position seems to be dead on now. So I'm looking forward to some real celestial navigation on the rest of the patrol and the ones that are still to come

Here are 2 images of my current travelled course and the now functional sextant reading.