How difficult / expensive is it to install a heat strip in a Dometic roof top AC? The unit I have has a mark on the knob for heat, but I suspect it was never installed.

Don't really understand what you mean by roof top duct unit .if it's a roof top unit and water is leaking down your vent then you have a clogged drain line .

What part about installation are you having problems with, and what are you installing it on? Do you have both upper (roof) unit & lower ( at ceiling) ? Is it a ducted system or blowing directly from ceiling assy?
Check this link as well , you may find 6300 series is same install.
http://www.dometic.com/enus/Americas/USA/RV-Products/Manuals-and-Instructions2/

I see what you see also in the manual. The difference is the Heat Strip package that part is optional and the 600212-001 with the optional heat strips becomes the new 600315-001 I talked to Dometic Duo-Therm and the models are the same but they both are not heat pumps but the new heat strip package comes on when told to go into heat mode when you change the thermostat to heat. The part you purchased will fit both model numbers you listed for your roof top air conditioner. Thanks, Sea Breeze

You can get lots of roofing companies in Houston TX. Just like CM Construction Remodeling, It is near to your location and helpful for you, get a fast response with reasonable price.

I ran across the same problem and found the kit to expensive. I gutted the parts added a transformer, contactor and thermostat. Unit worked great and cost about half the price.

Hi, like most of the RV roof tops, they will have a manual showing the type and wattage of heat strips you're system calls for. It comes as a unit, pre-wired to fit and set in place. You should have no problems installing a heat strip kit, but make sure you get the right one that the unit calls for as they do make several models. Very self explanatory. Also, be sure that you're system calls for a heat strip package?? Good Luck, and if you need more help please contact me. Very easy to set in place and plug in. Sincerely,
Shastalaker7

Heat real hot? Is this a heat pump or do you have a heat strip installed. Get a regular thermometer and put it in the outgoing vents of the unit. Register the temperature there and then the outside temp. If it is cooling at 17 to 23 degrees below ambient temperature, the manufacturer says this is fine. Try cleaning the condenser out on the top of the rv. Gently hose the rear condenser out until all the dirt and grime is removed the unit. Try cooling again and see if it is better.

maybe the capacitor for fan was already busted try to check that.

Does water drip out of unit on the roof while running ? If you look under the unit on the roof, you should see one or two little drains sticking down., that may be plugged up. Piece of wire or something and poke in the holes on the sides of those drains to make sure they're clear. Usually there is a plastic/nylon pan on roof unit to catch that moisture, and drain it off. Could be overflow because of the plugged drains, or I have seen the odd one crack but not too much recently, mostly back 5 yrs ago or so they had a run of them.