Cargo trailer plumbing

Who has some experience with mounting the tanks for a cargo conversion? Any suggestions on methods and locations. I am planning the fresh water to be built under the counter and need to figure out the black tank placement. I will be doing a shower and toilet. Also, is a grey tank necessary or does anybody just use a larger black tank? I bought a camper for parts and appliances and hope to reuse the tanks and such.
I am using a 7x16 v-nose double axle. I am a bit worried about how low the tank drain will be. The trailer is for camping and hauling my dirt bikes, so offroad sites are common use.
I am using a 7x16 v-nose double axle. I am a bit worried about how low the tank drain will be. The trailer is for camping and hauling my dirt bikes, so offroad sites are common use.