... He told me breaker panels can't be in the bathroom and they have to be 6 feet away from a water source. ...
This is simply not correct. He is quoting from a part of the code that applies to a different occupancy, not an RV. Here's what the 2008 code has to say about it in an RV situation (Section 551.45B):
The distribution panelboard shall be installed
in a readily accessible location with the RV in the
setup mode. Working clearance for the panelboard with the
RV in the setup mode shall be not less than 600 mm (24 in.)
wide and 750 mm (30 in.) deep.Exception No. 1: Where the panelboard cover is exposed
to the inside aisle space, one of the working clearance
dimensions shall be permitted to be reduced to a minimum
of 550 mm (22 in.). A panelboard is considered exposed
where the panelboard cover is within 50 mm (2 in.) of the
aisle’s finished surface.
The code recognizes that panelboard location in an RV is necessarily limited by the tight quarters and odd spaces specific to that environment.
With all due deference to my good friends (and electrical experts) Len and Melbo, I believe that they, too, are thinking of parts of the code that apply to other occupancies and not RVs. The above section contains the only specification for panelboard location and access in the RV environment, and, as you can read, it does not proscribe installation in a bathroom or require 6' of separation from water sources.
That said, article 408.37 does require that panelboards in "Damp or Wet Locations" be installed in compliance with 312.2, which basically requires a weathertight enclosure. Note that "damp or wet locations" has a very specific meaning, and just being in a bathroom does not qualify a location as damp or wet.
So unless your installation violates the working clearance specification defined above, or the panel is actually exposed to water, it is neither illegal nor code-prohibited in an RV.