Atwood Water Heater Relay - Stuck Closed Again

measure the resistance of the heater / element which will give you how much current it is likely seeing (after some math). Then let the heater run a bit and do the same measurement again and see if there is any difference. Perhaps as the element heats up, it gets less resistive such that it results in more current... not enough to blow the fuse but enough to overheat the relay contacts to the point of "melt" after a number of runs / cycles.

side question, what is the part number of the relay?