A couple of different opinions to some of the above based on my research. This is what I did on my RV-10:
I installed my VOR/ILS antenna using a cat whisker antenna from RAMI at the top of my VS. I have heard too may people say that placing it in a wing tip has resulted in signal loss during approach turns due to shielding of the aircraft during the turn.
I installed one bent COM antenna from RAMI under the fuselage behind the baggage bulk head, and the other COM antenna on the top of the tail cone. The antenna below the tail cone doesn't always work well when on the ground, but can be stronger than the one on top when cruising.
I used a single antenna with a GTX 45R transceiver for Xponder/ADS-B out/ADS-B in. I used a stubby rod antenna from RAMI for this and have it coming up in the center tunnel. Try to place it where it is as far as practical from all 3 landing gear legs, as they can shield the antenna, but also be aware of the Garmin coax lengths (min and max) when locating it.
I placed the 2 Garmin GPS antennas on the tail cone, offset from each other in both the longitudinal and latitudinal directions. You want to keep as much distance between them as you have space for, since one of the failure modes of these antennas (they have electronics built into the antenna) is oscillation with significant RFI with can disable the other GPS antenna if it is nearby.
I also installed the ELT antenna on the tail cone near the VS, with spacing in front of the VS per the ELT mfg. recommendations.
No antennas installed on top of the fiberglass cabin top per discussion with Van's, as they very firmly told me there should be NO additional holes drilled in the top that are not specified in the plans, since the cabin top is a key structural element to protect passengers in the event of a roll-over.
It was recommended to me by Aerotronics to try the G5 using just the internal antenna within the G5. I tried it and works beautifully for me. If it causes problems, you can always add the small puck sitting on top of the glare shield inside the cabin later.
Keep longitudinal distances between the antennas as far apart as possible while still maintaining adequate ground plane around each antenna. Garmin has a document with their view of minimum distances (written for certified aircraft).
This question comes up about every 1-2 years, and there are always of number of differing opinions in response. I'm not saying that I'm right and the others are wrong, as many builders have used different configurations successfully. Just offering how I implemented mine and some of the reasons/logic for why. I guess I need to develop a clean single page pdf showing all dimensioned locations to offer in response, but I'm away from home/hanger now and can't currently access my sketches put something like that together.
Hope this helps a little without further adding to the confusion.
RV-10 First Flight April 1, 2020.
Phase 1 Complete
VAF Dues Paid (plus a little more as the site is worth it.)