With a 2005 Sprinter, I'd suspect it's 30 amps. If the power cord is "about the same diameter as your old trailer, it's probably 30 amps. Few trailers were 50 amps that long ago and the power cord is nearly double the diameter of a 30 amp cord. You'll know it if you're hauling that heavy thing around to plug in your trailer. That's only speculation on my part that your trailer is 30 amps, but probably a good guess.
You might want to verify the function of the GFI in the trailer. If it's OEM, it's 12 years old and may be faulty. To isolate which circuit is causing the problem, I'd turn off all the circuit breakers in the trailer and unplug all of the 110 volt items. Plug the trailer into your home outlet and turn on the trailer main breaker. If that trips the GFI in your house, you've got a problem either with the trailer power cord or the power distribution center. If it doesn't trip the GFI, then turn on each trailer circuit breaker, one at a time. If it trips with a specific breaker, then focus on that circuit. If it doesn't trip with any of the breakers, then start plugging your 110 volt items into their respective outlets in the trailer. Eventually you'll find the one item that causes the GFI to trip.
Good luck, and think things through as you move from "nothing powered" to "full power"... Somewhere along the process you'll find the "problem circuit"...
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS