What Size/Type Fuse?

Without the schematic showing the fuse(s), it is difficult to guess at what is intended/required.

Going back to basics, fuses and circuit breakers (breakers are many times a better choice--Allows you to reset the circuit without having a bunch of expensive fuses on hand, and also very handy for an on/off switch when servicing your electrical system--And sometimes used to shutdown all the electrical loads when putting RV away for winter.

And fuses/breakers are there to (mostly) protect your wiring from short circuits/overheating/starting fires. The first question is what is the AWG of the wiring you wish to protect. A couple of simplified charts:

https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm (National electric code--Tends to be more "conservative")
https://boathowto.com/wiresize/wiresize_tables_abyc.pdf (marine ABYC--Not in conduit)

For example, if you have two "100 Amp rated" batteries (12 volts @ 100 Amps) in parallel, then you could draw 200 Amps (starting circut, large AC inverter, etc.). For a 200 Amp circuit, you look up the insulation type (temperature rating) and find the minimum AWG cable needed to support that current.

If, however, you have 200 Amp supply, and 100 Amp wiring to your largest DC load (i.e., ~1,200 Watt AC inverter), then you could use smaller wire and smaller Fuse/Breaker to protect that circuit.

From the Lithionics website:

https://lithionicsbattery.com/product/12v-125ah-g31-battery/
https://lithionicsbattery.com/wp-content/uploads/2022/04/74-505-12V130-G31LRBM8-Battery-User-Guide-RE.pdf (user manual)
For an RV, generally, you have loads that run for hours, and don't do a lot of high discharge/short time frame current draw (maybe starting the RV motor/heating&starting a diesel engine, etc.)...

Designing for 100 Amp draw would take a 130 AH battery to zero state of charge in a bit more than 1 hour... Normally, for "average" current draw, I would be looking at, for example (made up numbers, may or may not work for your needs):

• use 70% of battery capacity
• 12 volts @ 100 Amp * 2 batteries in parallel
• say 5 hours per night, for 2 nights (no sun) operation or 10 hours total
• 2 * 130 AH * 70% of battery capacity = 182 AH planned usage
• 182 AH / 10 hours of use = 18.2 Amps of average current draw over 10 hours
• Say 1,000 Watt AC inverter: 1,000 Watts * 1/0.85 AC inverter efficiency * 1/10.4 amps = 113 Amp max continuous draw (take your 2x batteries to near zero state of charge in ~2 hours at full load)

Once you have figured out your loads (average/max/surge)... Then designed the wiring to support that. You can look at fuses/breakers. Here is a nice discussion about how to pick the proper fuse(s) and circuit breaker(s) for your DC power system:

http://forum.solar-electric.com/discussion/353232/oversized-wire-and-breaker

Note that "normal loads" rarely operate at 100% continuous current rating... Heaters, Gym Lighting circuits, etc. can. And charging a battery bank is another (charging battery bank @50 amps for ~4 hours in your case--possibly). Typical North American fuses & breakers are rated to blow at 100%+ rated current (may take minutes/hours). And not blow at 80% or less of rated current...

So if you had a 50 Amp branch circuit running to some DC loads, then a 50 amps rated cable/fuse/breaker would be typical solution.

However, if you where charging your battery bank at 50 Amps, the NEC continuous load derating of 80% would be suggested to avoid nuisance tripping of fuses/breaker (i.e., 50 amps trips after 2 hours of charging):

• 50 amps * 1/0.80 NEC derating = 62.5 Amp minimum suggested rating for battery charging (round up to next standard size of wire+fuse) 

-Bill