I've been to various countries in the list (more than once in some of them). I don't think you're going to find wild camp in some of them.
I've been to Thailand, Cambodia, Laos and Vietnam and most of the area you could do some form of wild camping are now National Parks and you would need permits and they usually "recommend" you to have a guide as a form of touristic attraction. (Basically they want money). :) Also there has been a great effort to clean up the countries after the war but there are still regions where the danger of mines is high. So you can't just wonder around. You also need to keep in mind the Fauna.
I've done various outdoors activities around SE Asia from climbing, canyoning, hiking/trekking and I found it's cheaper and easier to pay for your guides then trying to do it yourself. We are talking about $30 a day for accommodation, meals and drinks and around $50 for a 3-5 days outdoor outing with guides and all gear you would need and food. You pretty much bring yourself.
I've been to Indonesia in surf trips (Luckily it's fairly close to my part of the world. :) ) and we climbing some mountains and volcanoes over there. You could do some camping but again you would need permits and guides.
I've never been to Malaysia, Singapore, or Philippines to say anything.
If that is not what you're looking for you might want to try somewhere else like the Himalayas where you can camp and trek as much as you want in valleys and mountains.
People in general are pretty friendly and despite the usual fight, prostitution and/or drug scam, you shouldn't find trouble there. I can tell you two occasions I experienced myself. Once in Thailand, we (me and 3 friends) got burgled and lost our bag with money. We slept in the beach that night and we are still alive. :) Another time 3 (girl) friends where tubing down one of the rivers in Laos. they had a bit too much and ended up miles away down the river. Drunk and Lost they started walking back up the river. Eventually someone offered them a ride and took them back to the place we were staying.
You might get burgled like us but I don't think you'd get killed but don't get my word for granted. :)
I think this part of the question is more suited for the Travel SO site.
Just a map to support my comment below. :)
All rights and whatever legal things to the right people... (I just copied form the internet. :P )
EDIT 2 -----
This answer is based on my experience there. I haven't been there for 4 years now. things might have changed.
The problem with wild camping is not because of landmines or population or wildlife. The problem (which I don't actually see as a problem) is that those countries rely on tourists to get some money into the economy therefore wherever can be accessed by people has been transformed into a "business" in a way that you would need to organize with someone to take you there.