Well, the fuse is too large for sure. The Runtz failure would be downstream of the device if current related.
I don't know your year of truck at all but want to take a stab at something, please no unkind words.
You said your sender is 0-30 ohm's. Is your gauge calibrated for a 0-30 ohm sender? I have worked on many gauges and the sender resistance for different models can be all over the place, i.e. my Packard was like 87-8 ohm's (no that number is not backwards).
Have anyone ever lifted the battery cable off the battery while the engine was running and if it's got an alternator? Hotrod informed us that that is a big no-no on modern alternator cars. The battery acts as a load and without it the alternator voltage goes way up and might fry a Runtz type device which I believe is a semiconductor.
