"a brush and a bucket" is a technique I learned from men washing their airplanes in the field at airshows. They would clean their $10,000 paint jobs with a coffee can and microfiber rag.
For a motorhome, get the best long handle car wash brush you can find. Look for long, soft and fine bristles. If you are where you can make a big mess, attach it to a garden hose. Wash a section at a time. Do big sections on cold days and smaller sections on hot days. The mud, dirt and grit will run off leaving a wet surface. Now, dry that section with microfiber towels so that the hard water spots don't have a chance to form. If you don't want to climb a ladder for the tall parts, get a microfiber floor mop from Lowe's, Walmart, etc. Get a half dozen replacement heads as they will get wet as you work around the motorhome. The whole idea is to get the grit, dirt and mud to flow off the bottom, then dry to prevent water spots.
If you are in an RV park where "washing is not allowed", skip the hose and do the same thing with a bucket. It takes about 5 buckets to work your way around the motorhome and, maybe, a sixth for the wheels. Dip the brush in in the bucket of water and, while it is dripping, raise it to the top and work down. Keep the sides wet and with water running down, one section at a time. If someone says you "Can't wash your motorhome here!", say your are not. You are only rinsing it off.
If you really have to put something in the bucket, try some Jet Dry rinse aid for dishwashers or RV Wash from Protectall. (
http://www.protectall.com/searchresult.aspx?CategoryID=3). But, I don't think these really do much.
A pressure washer does not do a better job. It blows off the top dirt, while the dirt electrostatically adhering to the paint has to be brushed free so it can run off. Try washing your car with a pressure washer alone. It does not work. (I once tried to set up a pressure washer and tank of distilled water for my Honda roadster. It didn't work.)
Our motorhome is nine years old with a 100,000 miles on it. I've been washing it out of a bucket since new. The only thing wrong with the paint is scratches from tumble weeds, rocks, a fire hydrant and few other things. There are some hard water spots that came from RV park irrigation sprinklers, but not my washing. The 3M front end mask is screwed up because I tried some "damp rag" shortcuts on a buggy trip to Newfoundland.
The best way I've ever seen for getting bugs off the front is to find an adapter for your outside shower so that you can run hot water into your hose that you have hooked up to the long handle brush. Warm water quickly dissolves whatever makes the dead bugs stick.
