Start at the start batteries, remove & clean all connections then reassemble & tighten. Take a photo first of the cable layout as it may not be bone stock, and you want it back as it started.
Sounds like you are running on alternator voltage, and unable to start off battery connections as they are now. the dash lights & gauges take full voltage only or they won't work. You're beyond that and not getting enough voltage to starter.
Be careful to inspect cable ends for corrosion. Occasionally I run into connections that are clean & tight but there is just enough corrosion inside the connection to kill proper passage of electricity for start/run. If cables are long enough, you can nip off the crimped end, clean the wire & crimp on a new cable end. If not long enough, you may be able to use that cable for another shorter run now or in the future, so don't chuck a cable w/bad end until its too small to replace your shortest cable.
We just went thru this w/an 04 here at the SoCal rally in St. George, UT. Coach needed boost to start, but had no lights, gauges, or SmartWheel functions; boost to start gave horn on full blast w/out pressing on anything. Removed start batt cables, load tested (good), cleaned & replaced connections & Voila!!, the coach fired up, gauges & lights went thru start check then run. These types of "12V" computer items (gauges, idiot lights, SmartWheel, VDC) need full voltage to run properly, otherwise they don't know what to think.