I would suggest you look at some online resources to get information about accommodations and ground transport options for Cambodia, Thailand, & Vietnam:

Rough Guides & Lonely Planet have good sites. For example, here's a link to the Rough Guides description of accommodations in Cambodia:

https://www.roughguides.com/destinat...accommodation/

I also suggest you look at some trip narratives for your potential destinations on crazyguyonabike.com: Bicycle Touring: A place for bicycle tourists and their journals

I found that touring in Thailand was just as easy as touring in North America or Europe, and in some ways, easier. Laos isn't as developed and has a much smaller tourist infrastructure and population density. I haven't been to Cambodia. A guy I toured with in Laos much preferred Thailand & Laos to Vietnam. I never made accommodations reservations (except for my first night when arriving in Bangkok or Chiang Mai). I don't think you'd have too much of a problem with a 40 mile limit in most of Thailand. I don't know about Cambodia. Like mev, I preferred Thailand among the countries I've toured through in SE Asia.

Around xmas & New Years, you're probably looking at an airfare of at least $1,600 each. Prices are about half that a few weeks later. You also need to see what the baggage rules are for each airline you're considering, esp. if you're bringing your bikes.