Most setups I have seen allow for both to complement each other. In other words if you needed 10a of charge current, you could get 7a from one source and 3a from the other, so in this example say the 7a is from solar and the sun sets, then the converter (charger) would take up the slack and provide all of the needed 10a. Keep in mind this is simply an example, the actual charge and discharge current is a moving target, similar as the solar panels output over the course of the day.
Also, it is certainly possible to setup a system so that only the solar panels charge the battery(s) and provide 12v to the RV, but in the case of having both enabled, (solar panels and a 120v charge source), they will both work together in most all common setups. ~CA
2010 GreyHawk 31SS