Since you will be buying a new truck also, I would first find the trailer that I really wanted and then get the truck that is MORE than capable of pulling it.
New or used is always a decision only you can make. Personally, I would shop around for a used one that is not more than three years old that someone has taken good care of.
Depreciation hit on a new one will probably be around 20%. However, if you intend on keeping this trailer for many years, I would probably buy new. I think you'll find however, that most will buy their 2nd trailer within a few years of the first one.
Financing a new one for 20 years? I wouldn't. Maybe five years. I would look for a used one that I could pay cash for or finance for three years at most.
Go to rv dealers and just look, look, look. Don't let the salesman talk you into ANYTHING until you have done your research. This forum, and others, are a good source for finding out the particulars of a certain brand/model and what other owners have to say about them.
Most of all, don't get into a hurry. Determine exactly what you want before you're ready to buy.
As for the truck, don't be misled about the mfg'ers "tow capacity" bull. It is only an advertisement gimmick. Payload and rear axle capacities are more important. For trailers, take about 12% of the trailer's GVWR for tongue weight. Trailer mfg'ers quoted tongue weight is normally much less that the actual tongue weight.
Hope this helps
Ron