Will this jump start a diesel truck?

Yes. 1500 amps handles most diesel engines, though extremely cold weather or larger displacement engines may push the limits. Test with your specific truck to confirm before you need it on the road.

How long does the battery hold a charge when sitting?

That depends on storage conditions and how often you use it as a power bank. Keep it in a cool, dry place and top it off every few months if it's your backup emergency tool.

Can I use this to charge my phone or run accessories?

Yes. The 15000 mAh capacity doubles as a power bank for phones, tablets, and small 12V accessories. Just don't drain it completely before you need it for a jump.