Can I use a deep cycle battery?

[bd]

Throwing parts at it could cost a mint and still not resolve your complaint.

Check all of the cable connections at the starter and perform voltage drop tests across the cables while cranking.  Check for voltage drop between the engine and cab sheetmetal as well.  Don't rely strictly on visual inspections.

What do you mean by...

The ground strap from the battery to the firewall looks like it's fine.

The battery ground cable should securely fasten to clean bare metal on the front or right side of the engine (either to the alternator bracket or directly to the cylinder head/block).  The ground connection between engine and cab should be through a 5/16" flat braided strap or 10-gauge wire.  The heavy gauge battery ground cable should not connect directly to the cab.

[srozell]

The heavy gauge battery ground cable should not connect directly to the cab.

I don't doubt you at all, but could you explain why?

[Dr_Snooz]

I had the same problem and it turned out to be my starter. I did take the starter in to have it tested before buying a new one though. Definitely test it before buying a new one.

[Irish_Alley]

a bench test most times wont show a worn or weak starter. and since the starter using most the batteries power it should have a direct path with least resistance. mos times theres a ground wire from the battery to the alternator bracket.that way theres little resistance the ground path doesnt have to go through motor mounts or through the small ground on the back of the passengers side head

[bd]

The heavy gauge battery ground cable should not connect directly to the cab.

I don't doubt you at all, but could you explain why?

The starter imposes the greatest singular demand on the electrical system, 150 - 250 amps while cranking.  Every electrical conductor possesses inherent resistance as one of its physical characteristics.  The net resistance of any given conductor is cumulative with length.  Connections - the surface area of contact between joined conductors - need to be adequately constructed to support the current demand of the load while maintaining some degree of protection against oxidation.  Hence, the starter cable run (the total combined length of the positive and negative cables, as well as, all of the interim connections) should be kept as short and direct as possible, with as few connections as possible, to minimize the voltage wasted (lost) across the cables and connections, due to cumulative resistance.   Effectively, the longer the cable run, the heavier the cables need to be in order to compensate for the resistance of the cables and supply the current demand of the starter.  The more the connections, the greater the potential for unwanted resistance.  Grounding the battery cable directly to the cab with a jumper to the engine is an extraneous connection between the battery and starter.  In addition, high current flow through the battery cables during crank produces a strong magnetic field around the cables.  The resulting magnetic field can induce unwanted EMI in the electrical system in proximity to the cables.  Keeping the cables shorter minimizes the effects of EMI.

And then there are considerations on the cab structure.  Steel manifests greater electrical resistance than copper or aluminum.  The area of contact between the terminal of a battery cable and the steel cab is limited by the surface area of the ring terminal used for cable attachment.  The greater electrical resistance of steel combined with the limited contact area of the cable could significantly increase the voltage wasted as heat at the cable attachment point and impair current flow.  The thin metal of the cab (approximately 19 gauge) is physically unable to adequately sink and dissipate the heat generated by high current flow, especially during prolonged cranking.  The cab sheetmetal at the cable attachment point could gradually anneal, shrink and eventually crack out.  Any surrounding flammable materials conceivably could be heated to the point of smoldering or even ignition. 

The reason the engine block and steel frame of the chassis are suitable grounding points is because of the shear volume of material.  The effective cross sectional area of the block and frame are far greater than the attaching cables, and the relative masses of the block and frame are able to sink the heat generated by common connection issues.

[srozell]

Thanks BD. I appreciate the clear and detailed response. It makes perfect sense to me now.

[Jon87V20]

Update: It turned out to be the starter after all. Cranks fine now. Thanks again to everyone for all the help!

[bd]

Glad you got it resolved.  Now, you can go about enjoying more road trips without worry!