The best place to carry extra weight would be to place the awning and poles somewhere near to the rear axle over the rear suspension either under a seat or similar if possible.
I certainly wouldn't be carrying it rear of the rear axle, until I'd weighed the van(fully loaded) and checked the axle loadings,to see how much spare capacity(if any?)there is on the axles..
Even when a Mh is within it's legal limit(eg 3300kg or 3500kg)it's not uncommon for the rear axle to be overloaded(...long overhangs,bike racks,rear boxes etc...)
You may find that you'll need to secure it further forwards....
If you gonna trudle around with a 25kg awning then suggest you travel with your tanks empty. We got an awning weighing in at about the same weight but now we choose when and where we are likely to use it! Ok we got roll out sunshade awning and some Lilliput Leisure windblockers to fit the awning rails. Have found that these windblockers provide enough privacy for sitting out and do actaully work. They cost a fraction of the price of an awning or leisure home type extensions and they weigh in at about 1kg. Have a look see and save some payload! :)