The most probable cause of your slow filling is down to a mineral blocked membrane. It can only be improved mildly by flushing. I’m afraid you might need to buy another membrane, that’s if the pre-filters are OK of course! The Merlin puts 4 litres of water to drain for every 1 litre of product. This is already a high flushing velocity. What a waste of water! It’s not the greenest system out there its just a drinking water system, if you want better performance you should try a commercial system.
For cleaning at this height I would be looking to use a 25ft pole, that should be ample and also allow you to clean facia and cladding too.
What we recommend to customers experiencing this issue in such low temperatures is to use a convector heater in the van. The small electrical ones you can get from hardware stores are ideal, these will keep a constant heat in your van and avoid freezing.
Flushing the membrane is a term we use to clean the build up of deposits from the membrane. To do this we recommend you open your drain completely so all water is running through drain. This will clean all calcium and other mineral deposits that can reduce the performance of the membrane.
Old painted timber frames can be a big problem, especially if the paint is white and peeling and we would generally suggest avoiding them. But you can wash them thoroughly and then blade excess water off the glass panes if you do have to do them. The pure water produces an excellent effect on cars and many places in the USA have already started this approach as they aren’t painted with water based paints and are durable. They withstand constant rain (especially if you live in the North West of England where we get plenty!)
As a company we do not endorse the backpack approach due to Health & Safety concerns. People still climb ladders with them and have to balance at height and the extra weight (20Kg) can cause falls. Our suggestion to solve the problem and meet your needs would be combining an Ecotrolley along with a 31ft Hydra Alloy Water Fed Pole, this should do a better job in general.