My routine has been to inspect the roof each time I clean, which is usually a couple of times during the season. However, this depends on where a trailer is stored and climate. I know that in high humidity areas, rubber roofs can get a lot of mold and need more frequent cleaning. With our move to Ohio, I'm lucky to have found reasonably year round storage for our new trailer, which should really help with roof seam maintenance.
I've always found it a good idea to really inspect everything and do maintenance when I prep the trailer at the beginning of the season. That's when I do things such as lubricate hinges, inspect and clean the areas around the water heater and furnace, sanitize water tank, etc. I also keep a can of dry lube in the trailer at all times and use it on the stabilizers as needed.
And, quite often, I'll do a few odd jobs while camping such as clean a window or tow, clean and organize the storage areas etc. but this works for us as we're retired and our trips can be very leisurely. Back when the boys were home and we were working, camping time was too precious to spend time working on the trailer!
I think the most important factor is to just regularly look over a camper with an "eagle eye" to catch any potential problems as early as possible.
Retired but busier than ever!
2013 Winnebago 2201DS - 6,200 lbs. or so loaded!
2007 Tundra 5.7