I'm not an expert on the model year being looked at and there has been no mention of transmission choice yet. I agree tow ratings are a combination of going, stopping and handling while doing evasive maneuvers.
But in reference to an 06 with a manual, the 373 gears are the same finial gearing as the old NV4500 and NV4600 gear boxes with 4.10 gears. When Dodge change transmissions the old school of thought was thrown out the door. old school of thought was 4.10 towing and 3.5 was a rare option... 3.73 is the new standard and most people complain about the RPM at 60. reference 2000RPM is about 62 MPH in a 6spd, 3.73.
My experience 5.9 06 DRW Manual. 100% Stock I have towed near 26K lbs with no issue for at least 80K miles, truck currently at 115K and no drive issues. I have never wished for 4.10. 7th gear maybe but never a lower one.
Here is a link to the capacity ratings.
I think any gear ratio will work fine for any Jayco 5th wheel, I just don't see 4.10 as being needed, if it is a great deal on the truck buy it. Gear ratios are easy to change if you absolutely hate it. I wouldn't walk away from a great truck due to finial gear ratio.
OP lets see some photos? is the truck Stock?