When in use, your trailer needs to be supported under the chassis frame so that the floor is rigid and the trailer does not tip. 

These stabilizing jacks are a classic accessory for vintage trailers. The aluminum die cast bases are vintage style but also strong, lightweight and rustproof. The chrome plated steel screw and handle extends the jack from 11" to 17". Each jack can hold up to 6000 lbs. And they are stackable when not in use to save space. 

Set of 4 for safety and stability.  Place 2 under your A-frame just as it disappears under the front of the trailer.  Place 2 on chassis frame rails under trailer near back bumper.  Make sure all 4 are placed on firm, level ground.  Level trailer before stabilizing with these jacks, then extend until they are very snug under the chassis; they are not designed to lift your trailer.