I use a battery-tender 24/7 to keep battery charged but after driving 25-30 miles it will not start if turned off.

NO , ALL diagnostic checks are not alright. Have a QUALIFIED MB tech perform a 'parasitic draw' test. Also , stop using the battery tender , let the battery run- down and have it tested for a 'bad cell'. Make sure , whoever you have doing the electrical tests is told about ANY body work that has been done. What made you start using the 'battery tender' and was that just shortly AFTER the body work you mentioned?