travel trailers are actually "tested" at the factory for 85 psi... That being said, 40 psi is a "safe" level...actually, IMHO, too safe for a newer trailer.
Trailers don't come with built-in regulators for several reasons, but, the biggest is that you want to regulate the flow from the pedestal spigot...that way you protect any external connections; i.e., hose, in-line outside filters, rinse hose if you use one on a "Y" off the spigot, etc. While a hose, you would think, should be fine at almost any pressure, that isn't true. Most hoses that are drinking water safe, will fail outside in direct sunlight, hot temperatures and high pressures, so you want to keep the pressure down in the hose to your trailer - put the regulator on the spigot, then connect to your trailer.
For a unit made in 2003, I would err on the "too safe" 40 psi limit, but, on units made after 2006 or so, I would say that 60 psi is safe to use. I have a Watts adjustable regulator that I have set at 60 psi and the water pressure in the trailer is just more "normal". I used 60 psi in my old 2008 210WBS and I use 60 psi in my new Cougar 5er and other than the sloppy installation of the connectors, I've never had a leak once I tightened up the connectors (as they should have been done at the factory...). The Watts regulator is also a high-flow regulator; many of the cheaper, preset 40 psi units not only restrict the pressure, but, restrict the flow. If you can afford it, you will be much happier with the increased flow and the adjustable feature would let you use 40 psi in your older trailer and when/if you trade up to a newer unit, you can adjust the psi up to 60 and at both settings the increase water flow of a "high flow" regulator will be a benefit, especially if you use the trailer's shower.