Reverse Polarity Fuses Circuit Question

Parallel means side-to-side, with the end leads connected together. So you'd have the wire from the converter, connected to one end of both 30A fuses, and the other ends of the fuses also connected together to one wire going to everything else. The goal is to have half the current flowing through each fuse, basically forming a 60A fuse. If the legs are somewhat unequal for whatever reason, the current won't be evenly split, and one of the fuses would be likely to blow even under non-overload conditions or when a reverse polarity battery connection is made, followed momentarily thereafter by the other one (which would then be carrying the full current). A single 60A fuse would be simpler and better.

Series connection means end to end, the same current passing through everything. Having multiple fuses in series is pointless. Having the fuse(s) in series with the output of the converter, however, is necessary—that's just saying that the wire from the inverter goes to the fuse(s), and then the wire from the other end of the fuse(s) goes to the rest of the world.