The Ram 6.4 is a fine truck. Please get the 4:10 gears...I'm not sure that you will find a crew cab capable of pulling your trailer, newer than 2010, for around 20k. At least one that isn't worn out. You may need to up your budget.
Diesel vs gas is a personal decision. I prefer gas trucks...I can afford a used diesel...I just don't want one...not enough pros on my ledger...