Jack stands are for a controlled environment - indoor level concrete floors, no wind, no serious work effort exerted on lifted chassis, no other vehicle or pedestrian traffic in the area, etc. etc. I know two people who have suffocated when their vehicles shifted while they were under them. Please do not trust a jackstand for anything that will put a person under the suspended weight - ALWAYS use solid blocking to chock up good and solid before putting yourself or others at risk. Google cribbing pylons for houses or search while adding Corp of Engineers as key words to see real-deal pylons.
Go buy yourself some 6x6 or better yet some 8x8 landscaping timbers. The idea is to make at least two pylons of cribbing that are pyramid in shape. If you choose two pylons place them inbetween twin axles or just aft of single axle. The base of the pyramid should be at least as wide as it is tall so plan ahead when you cut the wood. Top cribbing pylon off with hardwood shims to take the full weight and achieve level as needed.
If the trailer is going to be raised indefinately you need to crib under the 'A' frame. Simply blocking up the tounge jack post is asking for trouble - wind or other forces can use the trailers length as leverage to twist it off its supports. I had the tongue flop off its block the first place I stored this trailer from the ground settling under the cribbing I used - Think big to spread the weight out. A 36-40" by 16-18" crib footprint I'd consider minimum under your 31' trailer toungue.
This picture shows my partner deciding where to make the dead axle graveyard - the cribbing is not as exact as I would like but what is not shown is that the second layer of 6x6 blocks are almost as long as the height of the lifted trailer frame; I was more concerned about side-to-side shifting than front-rear. I've re-railed rail cars and moved 40 tons up over and back over track to be reassemled for towing but the picture shows enough for you to get the idea. Just think it through, don't take any shortcuts when actually lifting the trailer please!
(and yes; I've named the low spot in driveway under trailer Lake Winnebago)