Not enough space for lugs on post of battery isolator

Another option is to get a short length of copper water pipe, flatten it in a vice, and drill suitably.

The bus bar or whatever simply needs to be sufficient for the current flowing through it, not for the theoretical rating of the starting battery. At least in my RV that would be determined by the master fuses at either end of the solenoid, or something like 175A if I recall. Failing that, the wire sizes give some clues as to what to expect.