There's at least one gas (petrol) pump on each island. Filling up your car, however is a painfully costly business out here. Always fill your car up to the max before you make the ferry crossing to South Uist or any other Hebridean Island.
Now Malcom (Scotlandscotour) maintains that you SHOULD fill up your car in those communities even though it is so expensive there in order to support the local economy. He definitely has a point here and there's no denying that the economy in the Western Isles is very, very weak. Without local demand, some services might disappear altogehter -- with the people offering them. That means, if you aren't willing to support the local economy, you needn't be surprised to find even fewer facilities during your holidays.
On the other hand, tourists have a budget. It is not necessarily they who should pay the bill for an ill-advised tax policy...
In the end, the choice is up to you -- the informed, responsible traveller.
It's quite possible for wheelchair users to visit South Uist
Disabled Toilets (list probably not exhaustive):
-- Lochboisdale Ferry terminal (Large Calmac Ferries to the Uists are fully accessible)
-- Kildonan Museum and Community Centre
-- Borrodale Pub, Daliburgh
-- Orasay Inn, Lochcarnan -- based on their self-description
--- Crossroads B&B, Mr.& Mrs. MacRury, Stoneybridge -- based on my own experience
--- Eriskay Self Catering House -- according to their own description