Question:
Does anyone know if the major hot tub manufacturers make hot tubs heated
with natural gas?
Answer:
If you had to shove a (low BTU) tankless WH in one I would expect at least
$1K upcharge. Inline electric heaters are small and fit inside the tub
shape easily, gas would be much bulkier no matter what.
If you insist, you might try plumbing a small gas tank WH in the crawlspace
or nearby and rig up some sort of recirculation system with it in the loop.
This would require a bit of creativity and rework. Definately not standard
and causes you to put gray water through the WH tank which might have its
own problems. It also adds >25Gal to the tub capacity requiring more
chemicals etc.
The only areas that might have cheap electric heating are those areas
served by hydroelectric generating plants. That's not much of the
country.
However, I agree with you about the potential difficulties of
retrofitting a standard hot tub with gas heat.
