I have a log home with no bathroom fans. In a low humidity climate this is not a mildew problem, but it would be a nice to vent the other unwanted gas. The easiest place to install one is in the tub skirt and duct thru the floor joists. I think having the blower near the tub is probably a bad idea, so I am thinking of having an in-line fan somewhere in the crawlspace. Are there any issues with this setup? Is it a bad idea to have the intake down low? Is it bad to have negative pressure in the vent ducts?
Only caution would be if it's actually in the floor, in which case you might want to consider precautions from liquid water going into it. Down low it won't vent moisture as well, but since that's not a concern for you it should not matter much - right behind the toilet bowl would appear to be your ideal intake position. Negative pressure is fine in ducts unless you are using a jet engine rather than an inline vent fan.