If you are trying the cheaper route, you could first try Craigslist under RV/camper, or locate an RV store in your area that has used tanks and other parts. You want one that is not any deeper than the depth of the trailer axle
, otherwise you risk catching it on something when off-road. That can ruin your day!
I installed an after-market gray water tank on my 13' Scamp
and reduced the outlet to ¾”. I then ran a pvc pipe close to the side of the camper, installed a pvc gate valve, then a short piece with threaded end and screw cap. I did that because it allows me to drain the tank periodically into a 2-gallon collapsible container. I can then empty it into a pit toilet when at national forest or private campgrounds, or empty it well-away from any dispersed camping sites.
For draining and flushing when at a developed dump site, I cut off the male end of the drain hose and replaced it with a female end. Both ends of the hose now have female ends. After draining the tank, I can then hook up the hose to the faucet at the campgrounds or later at home, and backflush the tank. You can be sure the 3/4” hose is flushing the tank with more force than any backflushing through a 3” hose fitting. Afterwards, I also pour a little liquid odor control in the tank.
Not sure what others do, but I don’t let the tank get full to 20 or 30 gallons. I drain it periodically so it doesn’t take long at all. The simplicity of the process allows me to empty the tank in a lot less time than those clowns ahead a me who take forever just to decide what to do, take forever to do it, and then sit in their camper doing god-knows-what for 5 minutes before moving ahead to let someone else dump.
So not to worry about getting behind us 3/4" hose people; worry about the other 3" yahoos who have what seems 500 gallon tanks.