Been trying to start my 24v sierra for a couple of weeks. It starts and runs fine if i push start it but it wont start on it's own

Have tried changing batteries and checking obvious etc but when i try to turn the engine over all the lights dim down and nothing happens? Could my starter motor be stuck? would this cause the ligths to dim? My voltmeter shows a big draw on the battery so wondering maybe there's a short somewhere.

Any ideas?

Cheers