I have a 1994 Ford F450 Super Duty. 460 gas engine (Class a Motorhome). When I attempt to start the engine it cranks very slowly. I put a brand new Optima Battery in for the chassis and 2 brand new deep cycle batteries in for the house batteries. Thought it was the starter, replaced that and still no help.
This vehicle has only 35,000 miles on it and it ran fine when I had it out 3 weeks ago. The lights are bright the battery tests at 12.5 volts. Also tried jumping it with another vehicle to no avail. Any ideas suggestions would be greatly appreciated.
Checking the voltage drop of the battery cables would be the next thing to check. The proper voltage drop for a battery cable should be no more that .2 of a volt. Any more you should be looking for corrosion, bad connection, or bad cable.
Make sure all battery connections are all clean of any corrosion and dirt, and that means at both ends of the positive and negative battery cables.
Look into these suggestions and let us know how you make out.