Electric heating options when on shore power?

We have 2 electric heaters for our 26' body TT. Usually only use one and the other is a spare. Depending on temperatures depends on if we open the bathroom door. Both heaters have hi and low settings so when we run both they are on low only. The biggest issue I've found with mine is that they don't hold a temperature well, I've actually opened the vent slightly so they stay on keeping temperature consistant. Both of mine arw cheapos and actually came from yard sales.