If you are not a member of a Credit Union, join one and start building savings.
If you don't have a hefty down payment, save and wait until you do.
IMO, buy the truck first. Personally, I would get a relatively new Ram 2500 Cummins turbo diesel. Others, of course, would recommend a Ford or GM product. It is your choice. Then look for a fiver to match the truck. Keep in mind you do not want the latest, greatest, biggest trailer for the first time. chances are, there will be something you do not like about it, and you will be wanting to upgrade soon.
There is a chance that if your down payment is not large enough, the Credit Union may want (or require) you to buy "Gap Insurance". This is insurance that pays the excess, if any, between what you owe and what your vehicle insurance policy pays in the event of a total loss. It isn't expensive, and it gives some peace of mind.