Soft & sometimes soggy ground

Instead of the full blown concrete pad just pour two smaller pads where your tires sit. I made ours about 3'x 8'. Another option would be two dig down below the frost line, dump a few bags of concrete mix in and level it off. After it sets, stack cinder-blocks up to ground level and then back-fill around them.

For the house, I cut a sheet of 1/2" plywood into 4 pieces of 2'X4'. I seal them with multiple coats of Kilt's and external house paint. I stack them 2 high under the tires. The bottom two that sink into the mud last about 6 years before needing to be replaced. I take the top two with me when we go camping. My landing gear rest on concrete cinder blocks.

if you just sit it there? it will sink. long time clear up to the axle.put some cement slabs down then the treated plywood on top.

Agreed, for the house start with a base of 1's & 2's and then top with 57's. We had over 18" of rise over 12' of run so we built a little retaining wall with railroad ties, a stone base, and thick walled 1.5" pipe 6' into the ground. We used 10" timber locks to hold the pieces together.

If this is a parking site at your home, a load of gravel is an easy way to make a pad.

Alternatively,
- you could just do short 2ft wide pads for the wheels to rest on (maybe another small one for the the tongue jack).
- Landscaping blocks can also be used.

Wood is OK for short periods but not ideal if its' going to be longer term.

I don't know about trailer leveling, but I had a problem leveling my motorhome on a muddy campsite. As I extended my front two jacks (left and right), they just kept sinking...even with wood blocks under each of them. A nice fellow camper brought me a long piece of wood about the width of my motorhome. We laid that down under the jacks so both would extend onto the same long piece of wood. That worked much better. I just left the wood there for the next camper.

The ground is pretty bad off in the winter & spring. Since this is going to be parked there a while. I was thinking getting 2 sheets of 4x8ft x 3/4 inch marine grade plywood & having lowes rip it down the middle long way to create a 2foot width then have them cut it every 2 foot apart to create a 2x2foot square.Make 5 pads out of the 2 sheets.

I've used thick boards in muddy campsites when we boondock. It works ok but it is a pain to pry up the sunken boards after the mud has dried. Don't ask me how I know this.

You could use flat rocks, if they are available. The advantage is that unlike boards, you don't have to pack them up and take them home.