Interesting subject! I have thought about this as well, but for different reasons, but I am slowly going toward your reasons as the body starts to complain.
Answer: Could you buld a second slide shelf and modify the slides at a welding shop to hold the horz tanks...that I am sure you can measure and figure out.
Now for the real situation: FILLING. I had a 1997 TC with H tanks. they could not be filled while in the compartment because the nozzel wouldn't fit due to the side of the TC and door. Solution: I went to a welding supply house and purchased a right-angle adapter that screwed into the fill and presented the fill nozzel person with a straight in shot to fill the tank (NO PROBLEM). I would always install the adapter before the service person arrived and just showed them where to go...then I said hang on while I switch it over...it took less than a minute and away we went, the second tank was filled...no fuss no muss! Of course I had proper wrenches and tools to do the job, made sure everthing clean and a little dry silicon on threads always helped for speed. I only had one place balk and I asked for the managger...problem solved.
I can see an issue with double stack the top one may be too high, even with the adapter pointing down and I would not want to supply a stool for safety reasons.
That is my story.
How about going to the fiberglass marine tanks a bit less to lift. Could be as much as 50% less...maybe! Check this sight for alum and fiberglass tanks.
Defender tanks
