I don't think that is a good idea. Fuel is relatively heavy with a high specific gravity of about 0.8 (much camping gear and clothing is closer to 0.25 or 0.3 at the most). Have so much weight at the far back end of the bike, and you will degrade the handling. The best place for such heavy stuff is in the centre of gravity of the bike, such as in a Bikebuddy under the downtube. More generally, weight is I think the biggest disadvantage of the Orlieb line, even if the quality is indeed superb. First of all, you are easily persuaded that because they sell both front and rear panniers that you actually need both - but often you don't. Second, they are pretty heavy. Classic backrollers are 1.9 kg, or about 20 % of the loaded weight of a set. With frontrollers the ratio is even less unfavourable. If weight matters to you, and if you think 36 litres is enough (I think it is for many tours), a set of Pacific Outdoor LTW small panniers only weighs just over 1 kg. Perhaps not as bomb proof as the Ortliebs, but they deserve a thought.
Willem