Sorry you are having problems with your coach door... let me see if I can assist you. Basically, any "hardware", such as fasteners, connectors, fittings, etc., (unless designed as a "proprietary" patented, one-of-a-kind special and unique-to-the-specific-application and nothing else will work) ...CAN be purchased at a good general hardware store. Please do not spend $300 on this without first photographing it so that the great members on this board can have a look-see and maybe help you.
Meanwhile, from what you describe loosely as a "bolt", I think you have simply lost a small, non-threaded retaining pin which connects the closer piston to the bracket. It is fitted thusly to allow one to pull the pin, disengage the closer and allow the door to fully traverse the hinge arc for ease of egress with large objects... like appliances.
If that is true, you can temporarily do the "McGuyver" thing: Find a short nail which will fit the holes through the bracket "capturing" the piston and thus re-attaching the two pieces so that they will still articulate to open and close the door. You can clip the sharp point of a soft nail off with a diagonal cutter to avoid potential encounters with the nail... a cotter pin or other similar hardware would also work.
Again, if this does not help, please shoot a picture of the two pieces which need to be rejoined and I will give it another shot.
Best of luck...
Jim and Fran in Central Florida
1989 Country Coach Sedona Mark V
Turbocharged Detroit Diesel 8.2L "Fuel Pincher"