Hoping to purchase a trailer/tank setup next week.

I've been doing car lots for almost 8 years without a tank believe it or not. Some of my new dealerships are a huge pain so I need to get a tank asap. I don't know anything about tanks really so any info would be helpful. We average about 350 cars a day and I'm not even sure how long 200-300 gallons will last. Also I drive a silverado but I've put a ton of miles on it and wanted to trade it in for a tacoma. Not sure how the v6 will do with a load so I'm thinking of getting another 1/2 ton.

I would not be as concerned with pulling it the V6 should have enough power for it. I would be more concerned with stopping it , be sure to get one with electric brakes. If you balance it properly the 1/2 ton should be fine pulling that but check the towing/load capacity of the vehicle. For just the tank and trailer with 300 gallons you will be looking at hauling 3500-4000 pounds.

As far as water goes. Hook to your new tank and have filling while you work. I would get a float vavle so not to overflow.