I could rear face our Marathon in my husbands work car, a Chevy Aveo hatchback! I didn't even have to move the front seats to do it. You just have to know how to get it in properly and take your time. And read the manual. THe Roundabout will only harness to 40lbs. Its outgrown rear facing when the child hits the weight limit (33lbs) or there is less than one inch of hard shell above the head. The harness should be just at or below the shoulders for rear facing.
A forward facing car seat is outgrown whe nthe child hits the max weight, the shoulders go over the top harness slot, or hte tips of the ears are level with the seat back. When even one of these things happens, the seat is outgrown and must be replaced at once.
That being said, the Roundabout is a good seat and as long as its less than 6yrs old (car seats expire after 6yrs), then I would purchase a higher harness weight seat for your oldest child (if he's forward facing), like the Graco Nautilus or Britax Frontier. These will harness the longest and then also booster so your son will be able to remain in a harness in a good seat for a few more years, then use it as a booster.
Then move your baby ot the Roundabout REAR FACING and leave her there until she outgrows it rear facing per the specs above. The AAP, SafeKids and NHTSA all recommend children remain rear facing until the max of the seat, or at least 2yrs and 30lbs. My 3.5yr old daughter is still rear facing at 37" tall and 28lbs.
Anyway, the logical thing ot do would be to purchase a new 'big boy' seat for your oldest, a Frontier or Nautilus. Then move your baby rear facing in the Roundabout until she outgrows it rear facing, then move her forward until she outgrows it forward.