Miralax once per day is good; also HEALTHY FIBER taken 2-3X daily(Amazon). These are both non-bloating, non gassy. Stay away from Metamucil or anything with psyllium - very gas inducing!!
Also, on those days you simply can’t have a BM, don’t hesitate to use a glycerin suppository or a saline Fleet enema.
A clean rectum with no gas is ESSENTIAL to avoiding radiation proctitis. Ask your RO if he is doing a Cone beam Xray before each treatment; that tells the techs if it is safe to proceed with treatment; this is usually called IGRT - Intensity Guided Radiation Treatment.