Rico:
I owned an IP 27 from 1991-2001 and I replaced the holding tank and the hoses. On these older IPs, it is a really simple job because the tanks are fully accessible. The IP27 holding tank is under the v-berth starboard aft side. This can easily be lifted out and a new one installed. I recommend that you place the tank with a new plastic one from Duraweld in Florida. They should have the dimensions.
So, yes, if you smell sewer, you have a leak, and this is not uncommon. The original tanks were aluminum and the sewage will etch thru the welds and they will leak. So, get a new tank and pull the old one and install new hoses. The total job should take a day or two.
Hayden