2001 Jimmy won't start, no crank

Recently having major problems starting my 01 jimmy. Sometimes it will start with no problems, usually it won't start no crank, no nothing. As if there was no charge in battery.(Brand new battery at full charge) brought the truck to gm shop, checked thoroughly said everything is running fine, starters fine, charging system fine etc. Even got the to disengage remote start, still no change. Funny thing is when at gm, hooked up to their huge charger, started fine everytime? Been asking other mechanics, usually told me starter? Or crank sensor? One even told me to run a secondary battery? Which I don't want to do if not necessary.(Haven't tried any yet) So after multiple towing and shop charges, still scratching my head. Please help, any information will be greatly appreciated.