I always use Noalox on the cable ends. It's a conductive grease made for electric panels. the stuff never dries out,keeps perfect conections. I work it into the bare cable before crimping ends on when making up new cables and never get the usual corrosion in the cable strands.
Definitely get the largest diameter cable you can get. I made a set of 3/0 cables for mine
Another problem may be happening here too. Are you making a whole lot of short trips and turning the engine off at each stop? Remembering typical social activities at that stage of life, I figure that probably applies
If so, you're batteries may not be getting a chance to get fully recharged, and it would act exactly as you describe with the starts taking longer throughout the day.
Diesels draw a lot of juice to crank the engine over and the glow plugs have a pretty hefty appetite as well. Little quick stops like for fueling, might want to leave it running so your batteries have a chance to recover ( you don't have to shut a diesel off at the pumps, that's for spark ignition engines ).
If you are doing a bunch of short hops, you may want to put the thing on a trickle charger once every couple of weeks to top the batteries off.
---------Robert