You might take a look at "Sodastream".
This is a system where you use a cylinder of pressurized CO2 to carbonate water yourself, and then you can add different flavor concentrates to make Root Beer, Cola, etc.
This would do several things for you. First, it uses 1 liter hard plastic reusable bottles with highly effective caps, so you should have better luck maintaining the fizz for the shorter time before it's empty (compared to 2 liter soda bottles). That is, the bottles are better at holding the fizz and have to do so for shorter time and fewer openings.
Second, for pure water (sparkling water) you could refresh the fizz at any time. Note you can't do this if anything other than the CO2 has been added to the water (for reasons similar to those for not shaking a container of soda before opening it).
And a bonus, especially for RVers, is that it saves storage space since the main volume involved is water which is already in your tank.