First, be careful about assuming that it is okay to trespass on someone's unmarked land even if it doesn't have "do not trespass" signs.
You are VERY likely to be run off, especially if you are camping on their land - which skirts the edge of most trespass laws if not crossing over. In a SHTF situation it is also very likely that landowners will be patrolling their land with more diligence and be a lot less forgiving of trespassing than in a normal situation. Anyone near a metro area is well aware of the problems of homeless squatting on land without permission and usually take a very dim view of it, even on someone else's land.
A LOT of land around me is posted. If I catch someone squatting on my land you can bet I will be running them off very rudely, after I look at their ID so I have their name and I will report them to the county sheriff (the local deputy lives just down the road about 2 miles from me on my mountain road) and then informing my neighbors on the mountain that there is a squatter going around camping on private land without permission.
In my state if you are hunting on private land without permission, whether posted or not, it is criminal trespassing - it is the responsibility of the hunter to know whether they are on private or public land or not, and if you have a firearm with you, a strong case can be made that you are hunting (there is a year round hunting season for such animals as squirrels). I am not sure, but I believe the same is true of fishing (although the land immediately to rivers and large lakes is technically owned by the state for some feet along the banks, you do not have permission to trespass to reach a river bank, although you can fish from it if you get to it from the river or lake side - there is no such thing as a private beach for a public river here).
As for being "stealth", I simply use earth tone colors, no bright colors if at all possible, and I use the lightweight camo nets that you can buy for duck blinds and such to cover my tent and other items. I put the tent well off a trail and more or less behind brush. To me, the rest is more or less common sense about keeping out of sight, including not building fires (I cook with a stove) and not making noise.
Generally, most private land is increasingly posted and fenced and gated. This is also true of a lot of public land too - especially the watersheds. We used to be able to hunt, hike, dirt bike a lot of public land that is now closed to all access because it is a watershed. WA state is really bad about that, but Oregon less so, just some of the more fragile watersheds are closed now.
So there is still a lot of public land that a person can camp on for a limited time, some of it is private forest land owned by timber companies who may or may not have their land posted, but they often do have it gated and posted at the entrance, which is sufficient by law and some do patrol their land periodically even though it is vast in size and heavily wooded. Some of these corps still allow hunting and other uses with permission, but many do not any longer due to liability issues and again, it is up to the person hunting to know whether they are on public or private land.
I would say that generally your best bet would be to find a landowner that does not live on the land you would like to camp on, and ask their permission to hunt it periodically. If SHTF then you have a foot in the door to ask them if you can camp there until things settle down. Have a backup to camp on public land somewhere too.