RV Queen to Regular Queen Mod Challenge: Floor Heater Vent

Are you going to sit on the end of the bed? If not I would just swap out the mattress and go camping. It's not like the end of the mattress is going to sag with only 5 inches of overhang and I don't think your feet will know the difference. If you feel you have to extend the platform you could staple and glue a plywood cleat under the end of existing plywood and then staple and glue the 5 inch extension to that. No need to mess with the lower structure then. Good luck and happy camping!