Bad battery causing low starting voltage
Bad exciter circuit in the alternator
Bad voltage regulator
Bad or failing starter motor
Bad load shed relay
Bad starter button/circuit.
Where would you suggest I focus my efforts to start with and how can I test/narrow down the problems to find the caus
Use
this post from kioolt to diagnose a starter problem as Scott_ indicates is likely.
Read
this post from rbm for a procedure to help determine what might be draining your battery.
You should perform kioolt's test first.
A two-year old battery should hold a charge longer than yours is. I believe your battery is defective.
I'd charge up the battery, measure the voltage which should be up in the high 12V to mid 13V range, connect it to the moto, disconnect the transmission ground and leave it for a couple of days then measure the voltage again. The voltage should still be in the high 12V range. range and it should start the engine. If it doesn't,
it is one of your problems. If it does then after a few days of being connected it doesn't, using rbm's test procedure would be next.