Thanks to everyone for all the great feedback! I guess I will do my due diligence and call a few mobile welders and see if anyone is willing to tackle it. If that doesn't pan out I will probably go with j-d's idea of a triangular fish plate and some pop rivets. If I make the fish plate about the thickness of the luan, then when I re-skin I will just not put luan over the fish plate and glue the filon directly to the fish plate. The pop rivet heads will make bulges in the filon, but I can live with that.
Someone doubted the joint was welded, but it definitely was with small fillet welds on the top and bottom, but not on the sides. To me the welds looked like just barely enough to hold stuff together in the factory while assembling the sidewall, and not beefy enough to be structural, but that is RV construction for you.
Someone else mentioned that screws might crush the tube, tear out, etc. That is my instinct also, but the manufacturer clearly didn't think so: the two horizontal tubes stacked on top of each other in the picture are held together by three 2.5 inch #12 (guessing) screws driven through both tubes from underneath. It looks like the bottom panel of the cabover was assembled in one piece using the square tubing, lifted into place under the sidewall tube framing, and then screwed to the tube framing of the sidewalls. The screws seem to be holding fine, which is surprising.
Someone else mentioned that the same weld on the other side may also be cracked. If I still have some energy left after fixing this side I may open up the weld area on the other side from the inside and take a look.
