C15 / Freightliner Century help

My batteries are dying overnite after a full days charge with nothing left on... When they are fully charged 1 out of 10 to 15 times that i try to start the truck it does nothing when the key is turned except illuminate dummy lites, turn it off,wait 2 seconds and try again,it starts right up.. The engine prot(ect) light on the dash is lighting up for 1 second as the motor starts(every time now), i never noticed this before so its possibly related.Truck runs fine and has for the last 2 weeks since the 1st dead battery problem and the engine protect lite started illuminating, i just wanted to get it home..I removed all 4 batts, cleaned every connection and had batteries tested, all good...Unscrewed the ignition switch and checked that the wires going to it had good contact...Crawled under and looked at starter, checked wires, seems ok....I want to replace the ignition switch and see if thats it...How do i know which one to get and what about the key? It seems its all one piece but i have little experience with these parts so your input would be appreciated..Looking online they seem to be about 20$....Any other ideas please throw them out there...If anymore info is needed let me know... Hate taking my truck to Freightliner if its something i can do..Thanks in advance

Chris

 

I checked to make sure all wires were tight on starter..one thing i did notice is the starter has about 1mm of play but all the bolts were tight i could see/get to...what exactly do you mean? does the starter only see 12v with the key in the on position?

 

I ended up taking the truck to freightliner and they didnt find anything at all, no codes,nothing...i told them i had checked batteries but they did it agian, twice, all batteries good,wasted some money/time there, they said its unlikely to be the ignition switch also,truck started fine for them but then when i get home i get low voltage warning on dash during cranking...... I checked the mag switch, it has 4 posts, 2 were negatives, 1 was always hot, other was hot only during crank, nothing loose...... i cleaned both ends of the main battery ground with wire brush made sure it was all tight.... as in the above post i found that the starter will move in its mounted position about 1mm or less, freightliner didnt think this was a problem, all bolts were tight otherwise....I need to go back to work so im taking a small generator with me and a battery charger so i dont get stranded again.... The truck is right at 600k so im debating just replace the starter since its the original but that kinda sucks to do if its not the problem...ugggggggg thanks for your replies john and future.....chris