Go with the 4.10 gears and the gas engine and you will be very glad that you did. People get mediocre results with a heavy load when the gears are too high for the gas engine. The diesel develops peak torque at less than half the RPM's so it is never a problem as with gas engines.
The last color I would want for a truck is black. It looks dirty 30 minutes after it is washed.
The back and forth with the salesperson is part of the game. I never talk to a salesperson but from the moment I step on the lot I ask for the sale manager or the fleet manager. It cuts out a lot of the games and negotiating tactics and it eliminates the salesperson's commission which will save the dealer $1500 or more and that is money you can negotiate down the price with when it is not committed.
I have a diesel truck and it is needed when towing more than 10,000 lbs. on a regular basis. If you have a 4,000 lb. camper and a boat and trailer that weighs 4,000 lbs. then for a total load of 8,000 lbs. the gas engine with the 4.10 gears will be more than adequate.
Check it for yourself by downloading the manufacturer's towing guide and compare the towing capacity numbers for the different engine and gearing options.
This is the time of year when it is worth the time and effort to find a 2014 model year truck. You can save $5,000 to $10,000 as compared to the same truck for the 2015 model year.
From your post it is obvious to me and therefore to the salesperson that you are anxious to buy the truck and this weakens your position even more.
I would also go to more than one dealer to check inventory and also use Google search and Craigslist to find a truck.