Fuel log
You might want to consider a simple fuel log. If you have a notebook in the plane, it's easy to jot down tach/Hobbs time and fuel added each time that you service the plane. This will allow you to do a couple of things. First, it will give you reliable long-term data, and second it will allow you to spot anomalies that could be the result of a fuel leak, carb malfunction, etc.
This requires no instrumentation or in-flight engine management technique whatsoever, just the discipline to record the data at the conclusion of each flight or when you service fuel.
Obviously, if your "average" fuel flow works out to 7.0 GPH, this is only a reference number, actual fuel burn is a function of SFC, which really translates to the percent of power you're developing as the other folks have noted in this thread. As they also pointed out, you'll be pleasantly surprised at how efficient RV's are!
Cheers,
Vac