Picture is perfect at all times on my Wireless. The trick is to move the transmitter closer to the monitor.
Buy longer camera & power leads, or cut & splice more wire on both.
I purchased mine on Ebay for $54, as a refurb. It works perfect & has "night vision" :B
For constant viewing in your case, tap a constant 12 volt source in the coach or connect to your running lights & drive with them on when needed. Move the transmitter to withing 6-8 feet from the monitor. The closer the transmitter is, the less chance of any interference.
Here is a WIRED unit that was posted a few days ago in another thread. I'm impressed with the number of positive reviews & cost of it.
AMAZON