Hi from Colorado: I'll take the below subfloor position. I've done both above (69 Globetrotter) and below (my Overlander).
I found the above floor tank created too much height for a comfortable toilet position. Worse, the drain port points straight down making connecting the sewer hose a hands and knees operation in the rain and mud. Worse yet, the drain valve and bayonet connector was too low and yep, got scraped off on a fuel station entrance. Worse, worse yet, you will have to run a vent line from the tank up to the roof.
Toilets are easier to buy at normal heights (16 - 18"). Below floor tanks drain from the side making dump valve location and connecting the sewer hose easier. The tank can be larger, so you can use some of the capacity for grey water, like the bath sink. You have more flexibility with the vent pipe. A disadvantage is you will have to fabricate some type of tank cover to protect them from road hazards and provide a compartment for heating the tanks.
Hope this helps.
David
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WBCCI #8607 VAC Region 11
KnowledgeBase trailer renovation threads: 69 Globetrotter, 76 Sovereign, 75 Overlander, 66 Trade Wind Such fun !
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