Dreaded won't start question

[TheBryceIsRight]

Details out of the way:

2004 Thomas Saf-T-Liner MVP
CAT C7

Before purchase had a diesel mechanic change oil and oil filter, coolant, and overall get her into running condition. And she ran fine and got me from KY to OK.

I'm going to first go with it probably needs the batteries charged.
Got the bus to it's resting spot in December and then cold weather hit hard and wasn't able to even get to the bus for almost 6 weeks. And of course then wouldn't start. And since then we've had plenty of below freezing days including lots of snow. But now we've had much more consistent warmer weather. So basically I don't know what would be the first steps. I have cleaned off the battery cables and made sure they were back on all snug and happy.

Common things I have read about not starting:
Low Battery
Solenoid for the Starter
Need to crank it longer
Heater grid not working
Fuel injectors / fuel pump issues

I will be going out and checking the battery charge this week. Right now voltmeter is sitting at 12. So hopefully it's as simple as needing a charge. But any other steps y'all think are worth pursuing, I'd love to know.

And side question about jumping. With two batteries would I still connect cables to the - and + terminals that are connecting to the bus, as in the cables that are leaving the battery housing?

Thanks!

[TheHubbardBus]

A 12V battery at 12.0V is almost dead. A fully charged battery is 12.7-12.8V.
No sense pursuing or investigating other possibilities until you've dealt with the elephant in the room.

[sportyrick]

your batteries probably discharged and froze, their done if that happened. Charge them and then bring them to auto parts store and have a load check done on them but it's probably time for batteries.

[HamSkoolie]

YEP BATTERIES
12 volts is 1/2 discharged and going below 11 will cause permanent damage. The deeper you go after that the more damage caused and the faster the battery will be worthless.

[TheBryceIsRight]

Welp tried to jump the thing. To no avail. So gonna bring them into the shop and see if they're salvageable or not. Let's hope they just need a good charging.

Thank you guys for the help! I'll be sure to post if the "good new" is simply new batteries. haha