Grinder.
A grinder will cut your welded luggage tray off in quick time. They are cheap for the cheapo ones at Harbor freight, or moderately priced at a hardware store. You can grind it with a grinding wheel, or attach a cut-off wheel. a 4 or 4.5 inch grinder will be perfect.
If your square bumper tube is strong (some are thin metal, some are thick enough to mount a bike rack to), then you can bold on a receiver hitch using two of the above u-bolts, which is jsut the square receiving part, or have it welded on.
For bike rack advice stop in at any bike shop. They can show you many varieties and you can even get advice about Rv's if they maybe know or have done RVs a few times.. Bike shop, not us.