In dealing with an illusive problem I had with our Cruisair air conditioning/heating units I found out from Dometic that when they get older some of the old fuse holders become a problem themselves - causing the fuses to blow!
After replacing several fuses, I changed out the fuse holder for a rather inexpensive circuit breaker that fit into the same hole and that solved my problem.
Perhaps consider going to a circuit breaker rather than trying to find replacement caps.
I located what I needed at West Marine, it was easy to swap out and now if there was a problem I don't have to hope I have some of the correct replacement fuses onboard.
Nick