anytime something is melting or blowing fuses constantly means you are risking an electrical fire that may destroy your entire ride by continuing to use that setup!
run a decent gauge dedicated power wire to near the sockets, then wire several individually fused sockets off of that. you could use a small fuse box like this: http://www.amazon.com/Blue-Sea-Syste...dp/B000K2MBPA/
i wouldn't allow more than 10 amps to each socket to be safe. some cars have 15 or even higher amp fuses for their outlets, but in reality the cigarette adapters do not make great electrical connections and can get very hot running much current for extended periods. if you can hard-wire things like the cooler and skip the outlets all together, then do. or you may consider using a higher quality connector like the small Anderson Power Poles (http://www.powerwerx.com/anderson-po...owerpole-sets/).
don't forget to also fuse the main wire close to the battery if it's directly connected.
reference a 12v wire gauge table like this to make sure you are using correctly sized wire for the main run and the individual runs (always go a step larger if unsure):