Running AC from Generator 150 feet away

I missed the part about the Champion generator. I thought I read that the OP has one of the larger Onans that might well be quieter than a Honda.

Sounds to me like you have a CHAMPION and don't like the noise. Buy a Honda and sit it right beside your unit.

You are basically pushing the limits for voltage drop with 10g wires if you limit it to 5% (fairly typical limit).

If the 150' doesn't include the 30' built in cord or there are additional internal runs from the generator to the plug, a poor quality connection that eats up some voltage or maybe the generator isn't putting out what it should (no load voltage often can appear fine but drops under load.

Suggestions:
- Check all connections to make sure they are clean shiny with no corrosion.
- If possible, move the trailer closer and see if the voltage stays up with just the 30' built in cord. (if not possible, is there something in the house with similar draw you can check to confirm the generator voltage stays up when under a significant load).

If all those check out, it's probably just voltage drop due to the distance.
- If there is excess cord coiled up, can you cut it down? If it's 5-10' probably not worth the trouble but if you can cut 30-50' out, it might make the difference.
- Switch to a heavier gauge cord.
- Consider a voltage booster (aka: autoformer). At 99v, it may still be marginal. Our's gives a 10% boost which still leaves you below 110v.

Use this as your step down at the rig. Lots of 120/240/480 transformers on eBay. They are easy to connect. Run SEPARATE wires in underground (buried tubing).

Why even put up with a five volt loss? Its silly...

http://www.ebay.com/itm/Acme-3-Kva-1-Phase-Transformer-Pri-240-x-480-Sec-120-x-240-Single-Phase-Used-/152539463032?epid=1319667323&hash=item23840f7d78:g:MrcAAOSw7GRZDmFx

Big fat wire. Aim for a 1% voltage drop. Move the generator closer.

I think that's too long for #10. The post above reports 104-108v. I would not be comfortable with that.

When the A/C was at 99 what was the source voltage? Seems like a larger drop than would be expected. Possibly a loose connection at some point. Extension cord connectors are notorious to cause trouble.

The other issue is why did the power fade with time? Is it source regulation? Connection getting hot and loose?

If this is to be used often I would hardwire #6 copper.

Yes, your wire is too small. I'm kind of surprised because I've run my air conditioner on 125' of 12 gauge extension cord plus the camper's #10 cord and it worked fine. My voltage stayed around 104 - 108 if I remember correctly.
I wouldn't run romex under the grass, it's not rated for outdoor use or wet locations. You would want to run UF cable in that application or better yet, run PVC conduit with individual conductors.
According to Southwire's calculator #10 should be large enough. So, maybe you have some other high resistance wire problem or a generator problem. If you run new wire I would at least go to #6, it's not much more than #8. If you went even bigger to #4 you would see a bigger price jump.

Do your calculations here:Southwire calculator