I feel the fuse on the battery (positive post) is a
SHORT PROTECTION fuse so I would put one that is twice your expected maximum current usage.
The best answer is to buy a $200
Trimetric metering system and 500 amp shunt and actually measure you usage and go from there. For anyone really wanting to know the status of their battery it is worth the investment. Especially if you plan to add more batteries in the future and more so if you want to add solar and or an inverter.
Since that is expensive I would say the hydraulic motors for the slides runs about 30 to 40 amps max and the Level up about 80 amps so I would say 100 to 200 amps. Worse case if you blow one of these and there is no short then just increase its size.
My recommendation is a SWAG "Sweet Wild $$$ Guess". However I feel the intention is to prevent a fire from a dead short not protecting electrical equipment from too much current draw.
Protecting electrical equipment from too much current draw is the requirement of the individual fuse on each power drawing line.