Discovered a leak, where is it coming from?

Those kind of leaks usually start up higher where rain can accumulate and enter the wall. Gravity pulls the water down through the wall and onto the floor.

It looks like you have another leak with the utility knife, the scissors, and the band aid. Red stuff leaking?

I'd suggest to inspect the walls in that corner and see if everything is solid and sealed. One image you posted shows the discoloration in the corner. Since this is a utility space that is enclosed, I'd suggest to remove wall paneling in that corner and inspect. You may be able to cut the panel along the trim of the enclosure and pry the whole panel piece out of the corner. That would allow you to reinstall easily. One of the small reciprocating saws is a great tool for this kind of job.

Water test. Dry everything out first and then start with the door. Have someone pour water at the door from a hose while you look for water with a flashlight from inside. If it isn't th door, try higher up. Keep searching by trying water from different areas. Inspect caulk above....that is the usual areas.

Pay to have it seal tested.

donn0128 wrote:
Pay to have it seal tested.

Yup, a pressurized interior test is the best for finding any leaks. I DIY it with a furnace blower. I should do a test this year just in case something has opened.

donn0128 wrote:
Pay to have it seal tested.

The whole trailer? I can tell you now it's not air tight. Underbelly is "sealed" but not air or water tight, although water would have a hard time getting up there. And there are holes in the floor for utilities going up and down from trailer to the "basement" area.

92DakotaHD wrote:

donn0128 wrote:
Pay to have it seal tested.

The whole trailer? I can tell you now it's not air tight. Underbelly is "sealed" but not air or water tight, although water would have a hard time getting up there. And there are holes in the floor for utilities going up and down from trailer to the "basement" area.

Folks on here know that most water leaks come from places far from where the damage is observed. Look at any and all joints and redo anything that is suspect. If in doubt, a pressure test is worthwhile. Of course there will be leaks out the bottom, but you would be looking elsewhere. If it were me, I would pull that water heater, check it, and redo the installation. Often the water heater area is not sealed at all.

92DakotaHD wrote:

donn0128 wrote:
Pay to have it seal tested.

The whole trailer? I can tell you now it's not air tight. Underbelly is "sealed" but not air or water tight, although water would have a hard time getting up there. And there are holes in the floor for utilities going up and down from trailer to the "basement" area.

the test is not looking for "air tight" and then you "point" at the one leak you are looking for..... if it was, you would likely blow out the windows during the test... :-)
You are looking for air leaks some where near or in likely areas close to the problem. A significant amount of air is "pumped" in via an enclosed fan in an appropriate opening from the outside. Then you listen or spray soap water around areas to spot air leaks in various areas (ie. joints, fittings, etc). My guess from your pictures is that the water heater / piping / pump has a leak somewhere allowing a small amount of standing water to sit in the corner where you found the result.

I double what westend says. More than likely that leak is up near the roof and running down the inside of the wall. That wall board is the same stuff they use in showers so it is somewhat water impervious.

Hard to tell in the first photo but are those staples rusted near the bottom? Could just be the wallboard soaking the water up, could also indicate water coming down.

Is that the rear of the trailer and if so is there a marker light above? It looks like the darkest part of the floor is in the center. Water could be coming down the inside of the wall and the plywood is soaking it up under the floor covering. I would think if the water heater was leaking, you'd have the darker part near it.

Seals look good around the lights nearby but have some marine sealant (3m 4000 UV) stuff on the way and lap sealant. Out at home and am at work when stores are open. The nails/staples right above the corner are dark but the trim just on the other side of the divider show no signs of moisture. One thing I saw up top is that the strip on either side of the roof that is screws covered by a curved strip of plastic that pops into place had popped out near the awning. I revealed around the lower awning mount bolts. Unsure if I should try the tops, just don't want to mess it up. Everything up top looks pretty sound. Going over the roof very hard when the lap sealant is here.

How much does pressure testing cost?

I had a leak around one of the bolts that hold the awning on. Look for signs of rust.