flat tire

Loosen nuts and then run good tire up a home built wooden ramp.

and loosen those nuts now, then retighten. some idiot at the factory put them on with a air gun. I put a dap of grease on the threads.block od good wood and a bottle jack.

Yes, practice first, you will be so thankful you did.

Like Matt above, I use a 4 way wrench and a bottle jack under a U bolt or shock mount.
My DW and I have this down to an art. She gets the tire while I raise the TT and unbolt the wheel.

I have a TrailerAid tire changing ramp for a flat on the road. You drive up on one tire and it lifts the other off the ground.

At home I jack up the frame and set stand jacks for tire or wheel work.

I have changed a tire with just a bottle jack on the frame but I was in a parking lot with little traffic and lots of room.

However you decide to do it practice first.

Like the built in Jack Stand Buster.

If you have wide spaced axles on your trailer, you may have difficulties lifting the flat tire off the ground. Using a bottle jack (with a flat piece of 2x4) on the axle next to the ubolts is one common way to lift any given tire. I use this method along with a jack stand for safety. This dual jack/stand combo would also work nicely on the road.
Powerbuilt 640912 All-In-One 3-Ton Bottle Jack with Jack Stand

Get a 4-way if you don't have one. Im going to use a bottle jack. I think you would need to get the good tire a long way up for leveling blocks to work.

Evanrem....Those yellow leveling blocks? That sounds like a great idea

I have found rolling up on lego blocks or some other ramp lifting the bad tire off the ground the easiest and most safe feeling.