Were it me I would keep my "IN" and "OUT" separate and not roll everything into an all-in-one pricey ADS-B 1090 transponder. It's not worth the $4,000 difference to end up at the same place at a maximum and less functional options at a minimum.
In your particular case you are already half way to compliance with the digital GTX 327 which is a perfect match for many out solutions.
You likely never fly above 18,000 feet. Mexico and Canada have not adopted ADS-B out requirements. Much less you probably have N numbers smaller than 12".
For your "out" compliance consider:
The Freeflight RANGR Lite series with built-in WAAS source which is digitally controlled by your GTX 327. This solution has it's own built-in altitude encoder in case you want to ditch your old grey code encoder. Freeflight has both 1090 Mhz (above 18K feet) and UAT (up to 18K feet) options.
Garmin GDL-82 in-line ADS-B UAT out with built-in compliant WAAS source. This neat solution requires no additional transponder antenna and transparently integrates with your existing panel and equipment. No matter how old your existing transponder and encoder.
UAVIONIX Echo out.
All the above also provide for anonymous mode unlike the GTX 335/345/45R or any Mode S option.
For your "in" consider:
For exact functionality of the GTX 345 (45R) on your aera 660 and tablet and smartphone running Pilot or Freeflight go with a Garmin GDL 39 3D. If you don't care about backup attitude then go with the GDL 39 (non-3D). The GDL 39 series work perfect with the aera 660. I prefer the portable units to the permanent GDL 39R.
For the same all Garmin solution with more functionality than the GTX 345 consider the new GDL 52 series which is basically the follow-on to the GDL 39 series. It adds Sirius XM into the mix and feeds both XM weather and music in blended with the ADS-B traffic and weather along with an AHRS for backup attitude (like the AHRS-equipped 3D version of the GDL 39). Again I would steer away from the GDL-52R remote unit in favor of the GDL-52.
Maybe you prefer a different app than Garmin Pilot or Foreflight. Unlike the GTX 345 option you are not limited to proprietary Garmin "in" solutions. So instead of the Garmin GDL 39/52 series you could choose to go with whatever favorite dual-band ADS-B in hardware solution you prefer like stratux, iLevel, Dual, Skyradar etc. Combined with your favorite non-Garmin app like iFly, FlyQ, Hilton, Avare etc. The point being you choose your inexpensive in solution that can be changed cheaply and easily in the future as technology changes.
Since you asked, if I owned your RV equipped as it sits today, I would do this: Order a Garmin GDL-82 2020 compliant "out" unit and a toggle switch for anonymous mode. Order a Garmin GDL 52 (non-R) "in" unit. Order a aera 660 to GDL cable with power to connect the aera 660 to the GDL 52. If you glareshield mount the GDL-52 then you are done. If you want to remote mount the GDL-52 order a cheap transponder antenna for it and mount it on the belly a minimum of 36" away from the transponder antenna. If remote mounting the GDL-52 and Sirius XM was desired order a GMA SXM antenna for the glareshield or under the cowling. The GDL-52 can be easily released from it's mounting plate and moved between your other aircraft for further cost savings.
Jim
P. S. Never ask an avionics shop if you should buy a new expensive piece of equipment to replace something that already works great and can accomplish the same job with some cheaper alternatives and less installation and wiring work.