My Jayco 345 probably isn't much different than your 321. The one by the steps should be your fresh water tank drain, it will have a plastic turn lever on it. The fresh water tank overflow drain will not have a valve and if you've never filled that tank you would never have noticed that drain.
The low point drains should have labels announcing their locations on the side skirting. My unit has 2 on the street side forward of the living room slide up under the skirting. They have vertical pipes with off on flow valves and are awkward to change unless you have 2 elbows on each arm.
The one by your battery compartment is probably the battery vent. Not a fluid drain.