Generator won't start

A good starter motor will read quite close to a short circuit across the terminals—a fraction of an ohm. I had a bad starter motor (since replaced) in the Onan generator in my motorhome. I didn't take the time to suss out precisely how the rotor was wired—it has a whole bunch of poles—but did note that the DC resistance between adjacent commutator bars was a few tenths of an ohm except for the coil or two that were open.

Your problem sounds like a discharged battery or possibly a poor connection between the battery and the generator somewhere (which could be on either the positive or the ground side). I'd guess the battery being discharged first. Note that it might be the chassis battery rather than the house battery, depending on how things are wired—assuming you have motorhome (with a chassis battery) and not a trailer of some sort.

I used to have to put a wrench on the end of the Generator and turn it a half to a full turn to loosen stuff up to get it to turn over.

Last year I noticed a rattle when starting so I pulled the entire generator out and yanked the cover. I found the bushing on the end of the starter motor as well as the gear were worn and slopping.

I replaced the starter motor and she runs fine now.

I removed the generator with a floor jack (with wheels) and some plywood.

I did the deed outside in the back 40 so I was able to dig a wide trench under the rig to allow the unit to clear the skirting.

It was a full day job but worth it.

While the unit was out I cleaned everything, serviced the unit, painted all the metal with rust converting paint and then a nice top coat.

It was a lot easier to work on the unit while it was out of the RV.

My unit was over ten years old when I did this. Now it fires up right away every time just like when it was new.

It was a lot of work but well worht it.

Your battery voltage should be in the neighborhood of 12.6V, not 12V. Charge the batteries. New batteries from the store are usually not charged.
If you don't have a charger handy, jumper from a vehicle while running.
Your ohm meter may have been to a ground post or the circuit is complete in the starter solenoid from positive to ground. We have no way of saying that has any bearing on the matter.
As Chris B states, try to turn the crankshaft by hand.

Any access to manually turn the engine a bit? Things can get jammed if it stops just right.

12.0 volts? That's near discharge.

is there a little local switch on the side that acts as an override shutoff.

i did some work on mine and accidentally bumped it. i have an onan it was on the bottom edge on the right hand side.