There are a few different Onan 2500 models. Here is the manual for one of them.
As far as I can tell, the installation meets requirements. There is reference to a long list of other codes which I did not pursue. There is no reference to using a fuse or circuit breaker in the starter cable.
Rating 360 CCA @ –18 °C (0 °F)
6.3.2 Battery Cable Sizes
TABLE 7. BATTERY CABLE SIZES FOR TEMPERATURES DOWN TO –29 °C (–20 °F)
Total Cable Length1 Cable Size
0 to 13.7 m (0 to 45 ft) 2 AWG2
14 to 18.3 m (46 to 60 ft) 0 AWG
18.6 to 24.4 m (61 to 80 ft) 00 AWG
1. Battery cable lengths are total lengths from battery to the generator back to the
battery and when using a total of 1000 CCA (Cold Cranking Amps).
2. A total length of up to 6 m (20 ft) may be used in warmer climates or when
battery capacity totals at least 1000 CCA.