HOW CAN I FIX MY ABI CARAVAN FLOOR?

I HAVE A MID 80s ABI AWARD TRANSTAR 2 BIRTH . I HAVE DISCOVERED THE FLOOR IS ROTTON FROM THE WHEEL TO THE BACK PANEL, THOUGH THE WORST AREA IS UNDERNEATH THE DOOR WHERE THE STEP IS FITTED..
CAN I CUT OUT DAMAGED AREA OF FLOOR? IF SO , THEN HOW DO I FIX AND SUPPORT THE REPAIRED AREA ?