I'm unable to find a good document on how the QOS works specifically on the RV routers.
So I can't say for sure how the "highest" queue works on the RV series routers, but normally in QOS if you for example use a priority queue ( ie LLQ/expedited forwarding) it normally gets total priority over all other traffic, up to a certain point (which you normally configure). After that it just drops the packets that exceed the limit.
Otherwise it could throttle the device which would not be good.
Other queues that doesn't use the priority queue tend to be guaranteed a certain percentage of the bandwidth but can exceed it if there is unused bandwidth.
So just because you have highest priority it doesn't mean that it can take all of an interfaces bandwidth.