This presumably would be for the controls of the fridge, and there's a separate fuse or circuit breaker for the 12V heater power?
10A might be reasonable for the fridge and furnace controls if they're the only things on the circuit. 5A is rather small and probably would blow if both were used simultaneously. Often the RV maker will put a little key to the fuses and their proper sizes in with the fuse panel or in other literature, at least if they're nice about things. Sometimes these lists/charts are a bit hard to decipher, though...particularly in regard to what circuit has what stuff on it.