Alex, what we (at Raritan) suggest is to add our product "KO - Kills Odors" to the holding tank at the rate of 4 oz. of KO for each 25 gallons of tank capacity when originally commissioning the tank, and after every pumpout. Simply add it to a bucket of clean water, then dump it in the head and flush it off to the holding tank. That should hold you until the next pumpout is due. Then, after pumping out the tank, flush the tank out with clean water, and add the KO again in the same ratio.
KO breaks down the waste and the toilet tissue and helps to liquify it, as well as keeping odors to a minimum.
See Mfrs. part #'s 1PKO22 and 1PKOGAL at the following link (scroll down - it's 5 or 6 down from the top):
http://www.boatfix.com/elec/rarclean.asp
On that same page, you'll see the recommended toilet bowl cleaner "CP - Cleans Potties" (part # 1PCP22) as well. Just squirt a little around in the bowl, and swish the inside of the bowl with your johnny mop every now and then - works real well and smells nice, too! Then flush the head and you're done. It is perfectly compatible with KO and may be used in systems that have Lectra/San or Purasan treatment systems, too.
The above are just suggestions; there's no requirement that you use them.
One caveat though, about KO - you need a large diameter holding tank vent line (3/4" I.D. min., preferably larger) in order to get air into the tank. KO, in order to work properly, needs plenty of oxygen coming
into the tank. The small 1/2" I.D. and 5/8" I.D. vent lines that are included by many builders won't cut it, and the KO probably won't work in those instances.
If your tank has the smaller 1/2" or 5/8" vent lines, don't even bother with KO. Instead, try the product "Odorlos". This product generates it's own oxygen out of the nitrates that are contained in the effluent inside the tank. It doesn't always last until the next pumpout however, especially during the summer months when it's hot. It may be necessary to add more to the tank at some point before pumping out the tank. BoatFix doesn't carry it, but many other places do. You can do a web search on ODORLOS; their website contains a list of places that carry it, by zip code. It isn't expensive.