First let me say that I agree with the previous responses that the Garmin G3X Touch solution is superior. However, just like buying an airplane, you have look to yourself and ask what is it that you want (what is your mission)?
If you are looking to invest in your RV and bring the avionics up to today's GA standards, increase the value, capabilities, enhance flying experience, safety, situational awareness, etc. then the Garmin G3X or Dynon HDX, GRT, or a few other very capable systems are the way to go. Most of us believe Garmin is superior and you pay the extra for having superior.
If you are looking to have just an engine monitor that provides the same engine information as the Garmin systems or any of the above mentioned systems, then you should look at other Engine Monitors companies. There are several companies that sell nice engine monitors in a small form factor but that is all you get... an engine monitor.
I have installed and used the Electronic International CGR-30 in a couple of planes, and for the money, it has been my go-to for Engine monitor only. There are a few other similar systems like the JPI EDM 830 series and Insight G2 that I don't know much about other than the display looks nice and it's inexpensive comparatively. There are so many options for Engine Monitor only, that you really have to dig in and see what you want. As mentioned above Grand Rapids (GRT), also MGL, AVMap, and a few others. I chose Electronic International in 2 of my planes because of a personal choice in their menu system and their primary focus on being just a engine monitor company... JPI is probably the other that I would choose. You get a lot of info in 1 small form factor.
When it comes to pricing comparison ---
- Garmin G3X - There is no real comparison in price - you are buying a PFD that has all your flight displays but after you add all the modules (LRU, Engine Sensors, Engine module, etc) with no Autopilot... just to get to your engine monitors, you are looking at a minimum of $9K to $10K (maybe more). Again, you get so much more than just engine monitor and if you go this route I bet you decide to go all in and add autopilot servos, autopilot controller and other options which may end up being more like $15K to $20k.. so no comparison to just an engine monitor.
- Electronic International- This system is pretty much all in at around $4000
- Insight G2 - I've seen them advertised for around $2300 but I'm not sure what all that includes.
- JPI EDM 830 - Larger display that fits in to 1 round hole - not sure if you have to purchase sensors separately but it's advertised for $2800.
- Several other brands that mentioned above are even less expensive but have much smaller screen and are fairly old technology.
My point in all this is that you should really look at what you really want. In my opinion there is no comparison between the 3 engine monitors mentioned above and a complete advanced avionics platform like the Garmin G3X or other similar systems. Engine monitor is just a very small piece of what they offer but you have to buy the system to get the engine information.
Hope this helps, good luck.