I'm in the same FEMA trailer predicament as you guys. I couldn't pass the trailer up for the price. It's nice to know there's other people out there in the same boat... or trailer. Mine trailer is a 2005 gulfstream cavalier, 32? feet. Same toilet setup and everything. It was never used and is mint.
Depending how mobile your trailer will be you could just install a freshwater tank and use a portable black/grey tank. The site where I keep my trailer has water and electric but no septic. I'm thinking about picking up one of these tanks. http://www.watertanks.com/category/108/
. This isn't ideal if you're going to be moving the trailer a lot since it will approximately weigh 2,000 lbs full for a 250 gallon. If that will be too much weight you can get a portable septic tote like this. http://www.pplmotorhomes.com/parts/rv-toilets-tanks/25608.htm
. The largest portable tank you'll find is about 40 gallons but it will work in a pinch.
Obviously, the best choice is to install tanks on the trailer but I have two issues with this. 1. the reason I bought the trailer in the first place is because it was cheap. If I install 3 new tanks on the trailer how much added cost am I going to have? Is it worth it? 2. Holding tanks can get heavy. I have no idea if the trailer is constructed to carry the weight of an additional 3 tanks. These were my main concerns with this route.
The formaldehyde issues in these trailers are real. My best advice is keep the trailer as cool as possible. Formaldehyde fumes seem to multiply exponentially as the temperature goes higher. I looked at the trailer I bought and another, both FEMA trailers. It was warm inside and my eyes immediately started to burn when I went in. I suppose a HEPA air filter will help also. I'm putting one in mine.
Hopefully this helps. I'm still learning also.