First, let's talk about wire. We are not cranking an engine with this battery, so it should never be tasked for currents higher than your converter rating. (OK, maybe for a second or so to start a pump, but any wire in the guages we're talking about, 8-14, can handle a zillion amps for a second). You can get away with #10, or if you want to be extra careful, #8.
You need a fuze in one of the battery lines, not at the fuze box. Usually these are isolated fuze holders that mount to the structure--wire comes from battery to holder, then another wire from the other side of the fuze goes to the main fuze panel. Why? Well, if there is a place for a big fuze on the panel, like 50 amps, that's fine if you never see a time when you'd remove the wire from the panel before you removed it from the battery. You never want to have a live end of a wire that's connected to a battery waving around--batteries have lots of energy and it will come screaming out the end of a wire that accidentally gets shorted to the chasis. You can even blow up the battery if the short lasts long enough. Anyway, the battery line fuze needn't be any bigger than your converter rating.
Don't forget that you have a charging line from the tow vehicle, so just because you've disconnected the battery and you're not on line power (the converter is not powered), you can still have a hot wire when you disconnect the battery, ergo, fuze in battery line. It can be in either the negative or positive.
Ammeter: NO! Unless the ammeter is in the battery line and doesn't add any wire length, you want a resistor in the negative lead of the battery. Then you measure the voltage across that resistor. One end of the resistor is grounded, which makes the lead from that end to the voltmeter totally safe. The wire from the other end (these can be #20 guage, really small, or even smaller--there's essentially no current in them) of the resistor sees only the voltage across the resistor, which is
remember E=IR where E=volts, I=amps and R=Ohms
so if you think you might pull 50 amps and R=0.01 Ohms, you get 1/2 volt
so you install a 1 volt meter and it reads 0-100 amps full scale. Now, if either of these monitor wires gets shorted to the chasis, you never see much current, so they are very safe. On the other hand, if you put the resistor in the positive battery lead and then got a short, the wires would melt (12 volts across them) pretty quick.
However, I recommend you skip the ammeter for the time being. It's really easy to add in later.
I'm not sure you need to send the fuze block back. If you fuze the battery lead using a separate fuze block with an individual fuze (probably easy to find one for the round glass type fuze), then surely this little fuze panel is able to carry your full 60 amps on its input side. I doubt you have any 12 Volt
circuit that needs more than 20 amps. I'm also a little puzzled by the limit because you can buy automotive fuzes in the new (not the round tube, but the plastic rectangles) to pretty high ratings. There's only two sizes of these things (as far as I know) and if the standard socket for the appropriate fuze fits, then it's got to be able to carry the current (notwithstanding the comments that will come flooding in about fuze blocks "we have seen" with #24 wires in the back). I've got two 40 amps in my panel for the battery input lines, and then 6 each 20 amp fuzes for the 12 volt
distribution circuits. The thing I like about the automotive fuzes is, they are easy to pull, so I can isolate a battery very easily. There is a main fuze for the converter, but since I'm running a 30-amp converter (the Caravel
is small and all the Sovereign
has is lights and the water pump, despite its size) that's no problem, either.
Gee, I haven't had a smoke test in quite some time....