You can have to much trailer but you can not have to much truck.
Buy the trailer first. It will determine your truck requirements and your camping spot requirements.
Start with a used unit for both trailer and truck. You can often find a truck/trailer combo for sale together. Pay cash, owing money on something you decide you do not like after 6 months is a constant irritation. You can sell a used unit for close to what you paid for it, maybe for more if you made improvements.
For $3500 you can have my 5er :), but I not the truck, and I will deliver for 1/2 cost of fuel.