Well, I did an all-public-transport circuit around Asia a few years ago, so I might have some useful input.

Not sure how cramped your particular camper was, but be aware than public transport in Asia is often sized for the average Asian. I.e. if riding in a van is trouble for your knee, then sitting on a Chinese-made bus or in a regional airline seat may also be problematic. Budget for first-class train seats and extra-legroom flights. E.g. in Vietnam, I paid extra for a tour from Hanoi to Ha Long Bay on a luxury van - but it was definitely a better experience than being crammed into a 23-seat Toyota Coaster.

China's domestic train network, the high-speed trains in particular, is quite good quality. For me the low point of the trip was the bus from Luang Prabang in Laos into China, but having arrived in Kunming, the rest of the transport was fine - even the local bus between Dali and Lijiang (I think the high speed train line has been extended all the way there since). Personally I would aim to take the Beijing-Ulaanbaator train as an adventure experience. Not sure if there are good overland connections into Kazakhstan/Kyrgyzstan yet.

In Europe, depending on how much you want to spend there, you can just get Eurail passes that give you almost-unlimited train travel across the continent for a fixed number of days. A rail-centric trip across Europe is certainly an excellent experience. Remember that you can easily rent a car in any European town for a specific daytrip. E.g. last year I was visiting my sister in Belgium for a couple of days, and renting a car from the airport to go visit some places that were not convenient by train cost me about ten euros per day. (Okay, it was a tiny Opel Corsa, but there were only two of us.) So it's definitely a legitimate strategy to not buy your own vehicle in Europe.

You'll definitely want more time in Russia than that, if you're going east-to-west.