A couple other thoughts..... I don't know if there is such a device,
but it seems to me you need to measure the current draw over a
sustained period of time. Something has got to be drawing more then 60A
at some point, IMHO. I may have guessed that you had a loose connection
or something like that.... but replacing the disconnect and having the
same symptoms reappear is .... troubling.

Have you inspected the other end of the line at the heater? I'm
thinking you might see some symptoms there as well, discolored/cracking
insulation or something.

In any event, I say you should be thankfull that the fuse is there,
instead of cursing it...!