Repairing the floor to my caravan

Hi iam new to all this so bear with me... I have a ABI Maruder sportique 500CT and i brought it last year. I noticed that it had some water damage on the nearside front and found my self replacing alot of the wood around the front windows and nearside window. NOT WHAT I HAD PLANED!! Any way sealed the water leak. Took the caravan out for the first time last week and had a brill time. When i got home and started to jack the nearside jack down the floor moved (alot) When i looked under there i noticed it was rotton near the jack bolts and most of the area around that corner. I have striped the in side of the caravan and iam now thinking about cutting out from front to back 3ft of the floor and replace it with new board. BUT i have a problem with what to make it out of iam thinking 12mm ply sandwich with poltstryene in the middle with a wooden surround for strenght ( hard to explain) Any help would be great but please keep in mind on on a buget?????;

Steve