The purpose of a fuse, is to protect the wire by melting before the wire does. Every circuit should be protected by a fuse. The fuse normally goes in the "hot" wire, so that when the fuse burns, the entire "circuit" (hot wire (+) / load / return wire (-)) goes dead.
There should be a hot wire from the battery to the fuse block - that wire should be protected by a fuse or breaker.
There should be hot wires coming out of the fuse block to feed various loads - each hot wire protected by a fuse in the fuse block.
There should be return wires from the loads, to the negative terminal on the battery. These do not normally need fuses, since they are protected by the fuse in the hot wire of the circuit.
Often, the return wires are omitted and rather than running them all the way back to the battery negative, they are connected to the vehicle frame, which is then connected to the battery negative. That way, the frame serves as the return wire for the circuit. This is poor practice, but it generally does work, and it's cheaper and easier than running a bunch of separate return wires, which is why vehicle manufacturers do it all the time.
Since the purpose of a fuse is to melt before the wire does, the fuse MUST ALWAYS be rated a bit less than the wire. So, say for #12 wire, which is rated at 20a, you would normally use a 20a fuse. The reason you can get away with that, is that the 20a rating of #12 wire is conservative - #12 can actually handle a bit more than 20a, so if the fuse burns at 20a, the wire won't be to the melting point yet.
The most common reason that wires melt, is that the fuse is rated higher than the wire. This makes the wire melt but not the fuse. The wire is protecting the fuse, which is backwards.
From what you describe, I would guess either that A) you've got something wired seriously wrong, or B) that you've got loads which are greater than the wire can handle, AND oversized fuses that don't protect the wire. I.e., a 10a rated wire, protected by a 15a fuse and more than 10a of load being fed by the wire.
A little 12v tester with a lamp in it, should not burn the lamp unless your wiring is screwed up so that when you use the tester, the tester becomes part of the circuit (and the filament in the lamp now becomes a fuse).
Of course, all of what you describe could also be caused by extremely crappy grounding - like...oh...using the frame or chassis as the power return path...
Current: 76 E-250, bubble-top, self-contained|couple of old Yamaha enduros
Previous wheelers: 41 Willys|78 FJ40|78 Bronco|84 Bronco|74 Ramcharger|78 Ramcharger|79 D150 PowerWagon|77 D100|79 D400 dually, converted to 4WD, utility bed, 10' Lance|75 Westy|69 Scout, RHD|bunch of others|bunch of bikes|couple of boats|couple of motorhomes|blah blah|so what|not my idea|just doin' what I'm told|wank wank|this space for rent|candy is dandy|but liquor is quicker