Tough to find, many times. Prime culprits are normally any of the perforations through the firewall or front floor. There can be lots of them. Wiring harness, throttle cable, parking brake cable, heater hoses, AC lines.
Of particular interest is always anything that enters the cabin that was done by the RV manufacturer and not the basic van build, as they then to no seal as well.
The doghouse cover can also be a big issue, especially if the RV manufacturer has put add on carpeting in the front over the factory mat. Anything pinched in the sealing area can cause a leak. You might even find a cover clamp that has come unhooked by a foot or vibration.
As evil a job as it sounds like, if there have been repeated wet carpet episodes, I would seriously consider pulling the seats and side trim pieces and remove the carpet and whatever other mats or padding is underneath. It takes forever for carpeting to dry and floor rust is probably the most common rust through place in vehicles. Best to get it opened up, dried, rust treated, and the leak area isolated all at once.