Guesthouse availability will not be an issue even at peak season this year. The reference to "camping" leads me to believe it is your first trip to the region.

There is a glut of cheap places in Phnom Penh, even with the demise of the Boueng Kak Lake area. Siem Reap is a little more, but head to Wat Bo area and there are US$15 guesthouses.

Vietnam is not as cheap, not as competitive, as the rest of the region but you will find plenty of <US$25 lodging. Food away from tourist areas is delicious and cheap.

I don't know about Vientiane, but Vang Vieng and Luang Prabang have plenty of budget accommodations. VV may not have too much extreme low-end availability during Dec-Jan, but still plenty in the US$15-20 range.

Not going to Thailand? Traveler's paradise right now. They seriously overbuilt accommodations in all price ranges, and there is real competition in lodging, food and transportation.

Get off the main drag. There are places to stay for much less than on "Pub Street," wherever you may be.

Forget about camping.