Japan on a budget

In the middle of planning our 10 days in Japan and I’m a bit stuck.

Everything we want to go to is west of Tokyo with the exception of Ginzan Onsen.

I have worked us back to Gotemba with a night there. The day after for sightseeing.

The plan is travel from Gotemba to Ginzan Onsen overnight (or nearby and get breakfast before heading to GO). We are just there to look around and use the day baths.

We would like to use the overnight trains but Google Maps is suggesting the longest part of the trip is by bus. I’m 6’ 3 and quite round. I’m also disabled so sitting on a bus isn’t an option. Not only that, going by bus would involve more money.

Any suggestions? Am I missing something?