Check to see if your trailer has the Equa Flex Suspension as an option (Yellow leaf spring hanger between the tires). If so, you likely can't use a ramp as the suspension will have quite a bit of flex and not allow a tire to come off the ground.
Regardless, a bottle or scissor jack directly under the axle, close to the flat tire will be sufficient. For added protection, set the parking brake on the tow vehicle and potentially set up blocks and temporarily lower the stabilizer jacks on the flat tire's side.
Also, with safety in mind, get a high vis construction vest and warning devices for changing a tire. The side of a 60mph road is no place to find out what you really need.
2017 Forest River Surveyor 247BHDS...
...Tugged by a 2016 F250 XLT FX4 SuperCrew, Shortbed 6.2L, 4wd, 3.73...
...Joined with a Husky Centerline TS 800-1200lbs WDH