You can get glue on RV holding tank heaters as well. They are thermostatically conctrolled and come in both 12VDC, 120VAC as well as combo unit's. Along with that are dump valve and dump pipe heaters but they are usually just on/off controlled, no thermostat.
2006 F350 Lariat FX4 CC 4x4 PSD
2007 KZ2505QSS-F Outdoorsman