I know you have an older camper, but what you are describing is the same way the newer ones are set up as well. See this off my 2004 T2499 camper. Here there is a junction box by the battery and a disconnect switch. But the fuses/circuit breaker/breakaway switch is still setup like you mention.
You should leave the fusing the way it is now as well as the breakaway switch. If the fuses are all corroded, then yes replace them and the fuse holder if it cannot be cleaned up.
Now what each does.
On the 7 wire cord to the truck, that 30 amp fuse protects the truck. If a short or overload occurs when the truck is running in the camper, that fuse will trip protecting the truck.
The 30 amp fuse in the big read wire heading to the convertor inside the camper, that protects the battery/convertor if a short occurs.
If you tie the 2 together with one fuse, yes the battery is protected, but a short in the convertor or wire feeding it will take out your truck as there is no fuse then in the 7 wire heading to the truck.
The emerg. break away is wired direct to the battery upstream of the fuses/dissconnect switch so in case a fuse blows, the breakaway still works the brakes in the event of a detached trailer. Now if a short occurs in the breakaway switch or the brake wires in the camper being fed by the breakaway switch, the actual breakaway switch wire becomes the so called fuse. The breakaway switch is only about 14 awg wire. Meaning if a dead short occurs and the battery has a lot of charge, the wire or switch will toast itself. Since this is a safety device, if something is going to fail, it goes down trying.
Hope this helps
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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