I am looking into portable waste tanks. I cannot find the individual holding tank capacities in the literature. The manual only says "Dual Holding Tanks (Approx. Gals.) 66". Is there a capacity rule of thumb? The portables I am seeing are significantly less than 66 gallons. I can only imagine at the moment an overflow incident. Does anyone make a towable version? The dump site is about a quarter mile away. I am considering a mini trailer for that purpose.