thanks for the help guys, looks like I am going to go with this, since with the framing under my camper, and the low ground clearance, putting a holding tank of any real size under the camper is about impossible.
I have a low profile new Thetford bravera toilet, to mount to it. should work just fine, just have to find a shallow grey water tank, or maybe put the tank under the sink and rig up a water pump to pump the shower drainage up to it???
this one calculates to a a 9.77 gallon holding tank
the shower pan measures 23"W x 35"L x 7.25"D.
The holding tank measures 15"W x 21.5"L x 7"D (3" Bottom Drain)

I am putting it in this, and yes I already replaced the missing window
