How to test cables. using nothing more than a volt meter and a partner.
First, Measure voltage AT THE BATTERY TERMINALS.
now move the positive lead to the Chassis battery disconnect or to the starter solenoid battery terminal. Should show battery voltage. Keep the negative on the battery, Have partner crank (or rather attempt to) the engine,, Voltage goes down.. You have narrowed it to that cable or cables (See note), Voltage remains high.
Move positive back to battery, Move negative to chassis. Should show battery voltage.
Repeat cranking experment, Voltage drops, you have narrowed it down
NOW. Move the negative probe to the battery cable (not the post but the cable)
Try again, if it drops, clean battery terminal If it remains high find other end of cable and clean THAT connection.. ALso place the meter lead on the lug there and try again, Should start but if not did voltage drop? Replace cable.
I have had to replace two wires in my MH due to deteroation, One was a 10ga line to a power strip for the radios and such in the cockpit. the other was one of the inverter cables.. Both were very clearly bad. The cable. NOT the lugs. (on the 40 amp I was able to fix it (Excise the bad part, Solder and seal) but on the Inverter cable.. I had a replacement in stock.
NOTE, if you go to the disconect solenoid you need to check both sides of it. (Solenoid could be the problem) I do not think the cable from it to starter is bad else Aux start would likely not work.. But check it anyway.,