We swapped out our 13.5 for a 15 shortly after we purchased our current camper. Ours is 30 amp and as long as we don't run all these at the same time, we don't have any problem.
Electric water heater
Air Conditioner
Microwave.
So, we run the water heater on gas all the time, and we've never had a problem running the air conditioner and microwave at the same time. But if we flip on electric on the water heater, and the water heater kicks on, it almost always pops the main breaker in the camper.
Same with an electric frying pan or an electric griddle. If Air and microwave are running, and we plug in one of these, it almost always pops. Considering we cook outside all the time, I usually run a separate normal 20 amp cord from the power post to the picnic table and never plug into the outlet on the side of the camper.