Why am I measuring low voltage when I connect this fluorescent fixture to an existing circuit?
Simple answer to your question is: 
All load fixtures must be wired in parallel, never in series. This means you need to feed each fixture with 120vac. You can use a single run of wire, but the feed to the next fixture must be wired so that the hot and neutral are always connected uninterrupted to each fixture. Practically speaking, the black hot wire coming from the switch must be connected to the black wire going to the next fixture as well as the black wire for the local fixture in the same wirenut.  Same goes for the neutral and ground conductors. This is parallel wiring.  If you still have a low voltage reading, then you have a defective ballast or an open neutral.