Some experts will likely provide more specific details but I'll tell you what I've learned from several people who work on trailers...
There is usually insufficient space under the floor to run any sort of sewer line from the toilet to a tank that is offset from the toilet. The sewer line needs to be several inches in diameter and needs to have appropriate pitch to drain from the toilet to the tank. In a home this means using over a gallon of water per flush to move the solids through the pipe. So a black water tank offset from the toilet has two main issues - insufficient space and uses too much water.
1) An option might be to consider a composting toilet. I know it sounds weird but those I've seen installed in RV's and used properly seem to work very well. 2) Another option is a macerator toilet - this allows a smaller pipe from the toilet to the black tank, allows for less pitch of the pipe but still uses much more water than a conventional RV toilet. 3) A friend of ours uses a cassette toilet in one of his vintage trailers. Initially I didn't like the idea but the great thing is he can "dump" the toilet in any rest area he chooses and doesn't need to find an RV dump station. May not be a bad choice.
Lucius and Danielle
1992 29' Excella Classic / 2010 Interstate
2005 Chevrolet Suburban K2500 8.1L
2018 GMC Sierra K1500 SLT, 6.2L, Max Trailering
Got a cooped-up feeling, gotta get out of town, got those Airstream campin' blues...