Cannot compare prices as we are far far away but I am a longtime happy supporter of LPG, it is a valid alternative fuel, cheaper, cleaner, and otherwise wasted during the oil extraction/processing. Pity it is not so supported as it could be.
I have been running various cars on LPG for more than 20 years now, a few installed by yours truly as well when I was in the trade.
Yeah, there are a number of drawbacks: cost of installation but especially QUALITY of the installation (I cannot stress this enough), fact that you have a second complete fuel system which requires proper calibration (another no no with most installers), a minimal loss of luggage space/spare tire (which can be a problem in the Micra, but not really on a large car like a Superb), range and refuelling is not a drama if you can plan it accordingly, and you have to schedule a couple extra tasks in the servicing (e.g. LPG filter replacement, and so on).
But the pluses are absolutely worth ... at the local prices, running the 280 on "live" petrol costs me more or less the same than my venerable P38 4.6HSE with its horrible gas guzzling RV8 per km. In other words, the ~12km/lt I make on the Superb (long haul) is compatible with the 5-6km/lt of the RR with a fuel around 60% cheaper than petrol - in my case.
I had before an A6 Avant with the 2.8 193cv and was averaging 8-10km/lt which was in cost terms what an A4 1.9TDI would cost, but with a different level of performance and sophistication.
Anyway, each makes its choice.
The Superb offers a nice hole for a tank, I recon a 60lt fits easily if not 64/66 (got to measure how deep it is), which allows for a good capacity and range. My LPG colleague/installer mentioned for my engine type (DNF) only Prins is available as an option, and a system would set me back around 1500 eur. Must consider this runs at a 5-15% proportion to petrol, unlike indirect injection engines which can be fully switched. The double injector engines cannot be converted to run on LPG, at least this I was told.
Even with this consideration, for a long run the advantage is considerable.
I do not see as a drawback the "losing" of a spare tire, I use the Micra for a runabout in town since 3yrs/36k km and the spare sleeps in the garage since then. Same for the RR and my previous Audi, except when I make a long trip and then in that case I load it up, yes it bothers and takes some space, but you tend to forget when the wallet thanks you ....
I still got warranty left on mine so for now I just go along, but will see in the future.
PS: to the OP, I think for the driving you need to do you will hate the iV with a passion. That small capacity engine on such a large body and FWD is not for that use. Today I made 420km in exactly the type of roads you describe, and the capability to 'point and shoot' given by a more than decent powerplant is simply almost exhilarating.
I hope I can remove the "almost" once I deal with the laggy pedal 🤪😝