Where are you located?
So you have no wheel/tire on that side but the brake drum is still there, correct?
I'm not sure about tax axle spindles, but I have replaced them on cars and trailers. On cars they usually bolt on, but some trailers have the spindles welded on, which would obviously complicate replacement. If it's bolt I would think on any competent RV shop should be able to do this, but I agree with the previous comment that a truck repair facility might be a better option. Is it that the RV shops couldn't do the work or wouldn't?
Someone correct me if I'm wrong, but I think Dolphins of that vintage used Mor-Ryde tag axles with Dexter brakes, so the spindles shouldn't be too hard to come by.
2004 National Dolphin LX 6320, W-22, 8.1, Allison 1000, Front/rear "Trac" Bars & Anti-sway Bars, Sumo Springs, Roadmaster Reflex Steering Stabilizer, 2005 PT Cruiser Toad