When battery is inserted, the internal fuse blows. I have tried changing it to no avail. Battery may have been inserted backwards at one point,

As Steve said . The fuse protects against installing battery cables backwards or backward hooked jumper cables .

NO
a battery with a higher crank specification only means that you can spin the engine for longer before the battery goes flat
blowing fuses is from a dead short in the wiring, battery connect up wrong, problem relays
basically the positive of the battery is through the fuse) connected directly to ground at some point

If I understand you have 120 volts but no 12 volts. Most likely the main 12 volt fuses or switches are bad/blown/off. I would start looking near the house battery and work out from there. Some battery charger/converters have internal fuses that can blow if the battery is hooked up backwards. The 12 volt fuse box usually has a main as well.
Don't forget to look for inline fuse holders and if this is a trailer, look on the tongue for 30 amp breakers near the battery that look like rectangular silver boxes. Some have resets on the ends and others are like fuses.

Hello Glen, Bad bad things can happen when you hook up a car battery backwards. If your car has a high quality electrical system it might only blow out one or two main fuses. If not, it can cause havoc on the wiring system and burn out the alternator, computer, sensors, modules, and stereo system to name a few.

check all fuses in all fuse boxes.
many parts, dont like that, and will blow a fuse, protecting these devices.
that was a big no no... (my job was to fix , that kind of damage)

The most common problem would be that it would blow the fuse, there is a 50-100 amp fuse, it may be listed in fuse box as alt, or batt.
the best thing would be to start and see if something does not work, there are changes of computer damage but most times the fuse protect that, again drive and see if there are any problems, did fuses pop?

Why was the battery replaced? Theres no reason the the 100 amp fuse to blow unless the alternator is shorted or you hooked up the battery backwards. I think the 100 amp fuse goes to the alternator. If it was hooked up backwards then you may have engine computer damage.

any time a battery is hooked up backwards anything that contains diodes will have to be replaced.unplug the radio and see if you can put the fuse in without it blowing.when you get the vehicle going have the alternator tested,it also has diode in it.good luck.

Start by dis-connecting the power lead at the amp, and securing it (where the bare wires are not touching ground) Now insert a new fuse then make the battery connection. If the fuse blows then you need to trace the power wire to find the short, or replace the entire wire, paying close attention to sharp edges and pinch-points that could cause this problem to reoccur.
If the fuse didn't blow, dis-connect the power lead at the battery then all of the other connections at the amp, reinstalling the power lead to the amp. With only the power lead connected at the amp and the good fuse inserted, reconnect at battery,and if the fuse blew then the problem is with the amp. and should be repaired or replaced depending on its value and the cost effectiveness of those two options.
If the problem has not yet been discovered, continue with tracing each of the remaining amp wires in that manner, as been described above.
Good luck...

Look For Blow Fuse LInks they are usually on the Positive Battery Terminal or in the fuse box(s)