Howdy, and welcome to iRV2.
Depending on how closely the old standoffs broke off, you might be able to grab the end with a pair of visegrips and wind them out. Dependent on what type of material they are screwed into behind the fiberglass rear cap, you might find a liberal soaking with penetrating oil helpful.
I'm sure someone here has ran into the similiar situtation and will have an answer.
Joe & Shelly, Justin, Tyler, Alyssa | Butler PA
2008 Tiffin Allegro Bus 43QRP|Cummins 425|Honda CRV