Good luck with the hitch, the DO35 is definitely a better unit. You've checked the paperwork and made sure it wasn't an optional upgrade I presume?
Those holes in the water tank are breathers, that allow you to fill and retrieve water without creating an airlock or vacuum. Ideally they should be plumbed high up nar the filler so that (1) water doesn't slosh out, and (2) so that nothing else gets into the tank.
If you have any doubts about what you've got, in terms of delivered state, clear those up before you go too far with your own mods.