27 is at the older end to be going to RMAS, but not unusual nowadays and the year he will spend there will be tough. My husband and I were engaged when he went through as a cadet and we found it hard. We've also seen RMAS life from the other end of the spectrum as he was later posted back there as a platoon commander and we had two wonderful years in Camberley.
You will be separated for that year - there is no entitlement to housing until he gains his commission. For the first five weeks of first term, there's no leave at all. After that, he gets every fourth (I think) weekend off. If you choose to rent a flat/house nearby - rather expensive in that part of the world - be aware that he won't be allowed home at nights. If they get the odd spare bit of time off, he could come home and visit you but the Academy prefer that cadets live in the platoon lines. In the second term, he'll get every fourth weekend off, and the same in the third term. Leave between terms is generous (usually three weeks) but one of those weeks in the leave following the second term will be spent on a cadet expedition. Although you won't get to live with him, there is support out there and you must impress upon your husband the need to let his Platoon commander know of any potential issues that may arise during his time there (if you or your daughter have any ongoing health issues, for example). The more the staff are informed, the more they can do to help - although all this will be balanced against the need for your husband to complete all parts of his training. Make sure that - wherever you are in the country - you have all the help and support you need from family and friends. A year is a long time in a marriage and there will be times when it all seems very difficult indeed and that's when you'll need people you can trust around you.
I'm sure you already know this - the regt/corps that sponsor your husband may not take him at the end of the course, so don't plan too much based on what and where you might be going after RMAS. During the year, he'll get the chance to register an interest in up to four cap badges, which are whittled down to two in the last term. He'll be interviewed by both regt/corps and then he'll find out who has accepted him. If he's lucky, he'll get a choice. If he's really unlucky and neither of his choices can offer him a place, one of the other cap badges will offer him a place - so don't worry too much! My husband went through RMAS sponsored by the RE and ended up joining an entirely different part of the Army because he changed his mind as he got to know more about what other branches of the Army can offer.
Once he's left RMAS, he'll go to his Young Officers' Course. From this point on, you'll be entitled to housing. Basic officers' housing can vary as much as soldiers. You usually have three bedrooms and the quality/standard can be excellent or it can be rather ghastly. After that - well it really differs from cap badge to cap badge but, believe me, it can be a wonderful life.
I hope all this helps - if you want to know anything more, just ask!