Can anybody tell me if electric mobility scooters are allowed on RER/Metro and buses in and around Paris. I am hoping to travel from London to Disney Paris and hoping to stay away from the park and use the RER or other public transport to get to the park
Paris Metro is generally not an option as these stations pretty much all required navigating stairs and or escalators.
Paris RER does have many stations that have elevator access to platforms and fare gates that can be opened for wheelchairs, strollers and other mobility devices.
The RER trains themselves generally required the conductor to place a portable, foldable ramp between the platform and the train itself for entry/descent onto & off of train cars. When waiting for an RER train, look on the walls for signs pointing to the direction of travel. Place yourself at the head of the train so that when the train pulls in the conductor can see you. Try to wave/indicate that you'll need assistance and he/she will pop out of the conductors cabin and place the ramp down onto the nearest car entrance to allow you to roll on. Then stay near the conductor's cabin so you can let them know when/where you want to get off and they'll repeat the ramp procedure.
Paris city buses are almost all wheelchair accessible now using the back doors which is where the foldable seats are located to make space for wheelchairs, strollers & mobility devices. It'd be recommended to have your tickets or passes in hand prior to the bus so you can use the pass readers or ticket stamping machines at the back doors rather than having to try to purchase an on-board ticket from the driver, which would be awkward.