a/c unit heating element?

Best way is a voltage reading with the battery 'at rest', meaning no charging or discharging for at least 3 hours.  Then compare the voltage reading with the colorful chart about 2/3 of the way down on this page:  http://www.windsun.com/Batteries/Deep_Cycle.htm

There are other ways, including expensive meters which measure amps in and amps out (like Trimetric).

My last motor home had 2 Coleman air conditioners with the heat strips.   The motor home had ducted air distribution and no matter if we were in heat or cooling mode, the system was very quiet. Note, the keys words are Coleman (which are much quieter than Dometics) and ducted air distribution.

My present pod has a Coleman, and if I did not live in Florida I would probably install a heat strip in it.  I believe my 178, which is on order, comes with a Dometic.  If so, I would choose a portable heater over the heat strip. So, I agree with all the comments.

Bob

Don't bother.  Everyone I have ever talked to says a small portable heater heats better and, obviously, much quieter.  The option also isn't cheap since you can buy 2-3 portable heaters for the cost.  If you can't install it you will also need to pay for that, too.  I recently discovered mine came with the "heat strip" option (although it wasn't activated) and I removed it while doing maintenance on the evaporator. 

http://www.campingworld.com/shopping/item/coleman-mach-mach-8-electric-heating-element/69629

This is the link to what I saw.