Should I travel with full holding tanks?

The quick answer, no. A gallon of water weighs 8.33 pounds. That’s probably a pretty accurate measurement for your gray tank’s contents. The black tank, with added human waste, is more likely in the 9+ pounds per gallon range.

RV holding tanks range from 30 gallons up to 70 gallons and even more. So let’s do a little math, based on the average tank capacities for three types of RVs (travel trailers, 5th wheels, and motorcoaches):

Travel trailers
Black tank: 30 to 40 gallons at 9 pounds/gallon = 270 to 360 pounds
Gray tank: 30 to 50 gallons at 8.33 pounds/gallon = 249.9 to 417 pounds

5th wheels
Black tank: 40 to 50 gallons at 9 pounds/gallon = 360 to 450 pounds
Gray tank: 40 to 70 gallons at 8.33 pounds/gallon = 333 to 583.1 pounds

Motorcoaches
Black tank: 5o to 70 gallons at 9 pounds/gallon = 450 to 630 pounds
Gray tank: 50 to 100 gallons at 8.33 pounds/gallon = 416.5 to 833 pounds

That’s a lot of weight!

The danger here – and this is much more a problem for travel trailers and fifth wheels where tanks tend to hang down from the RV’s frame is that you have a lot of weight that is putting stress on the tank and straps or hangers that are holding it in place. Motorcoaches are a bit different as their tanks tend to be more integrated into the overall construction, but you’re still looking at a lot of weight on one side of the coach.

That weight and stress while traveling could cause the tank to fall, causing severe damage to your RV and waste handling system (tanks, pipes, and connections).

The other issue to consider is your towing weight if you have a towable RV. Adding in that 600 to 800 pounds or more could be too much for your tow vehicle’s safety rating.

Yes, if you’re coming off of some boondocking, you may not have a choice other than to travel a while until you can dump the tanks. But your absolutely first stop – especially if you anticipate the tanks being pretty full – should be a dump station or other facilities such as a private RV park, truck stop, or even municipal waste treatment plant (some allow RV tank dumping for a small fee).

What about ice in your tank to help dislodge solids and anything stuck on while you travel? Well, we debunked that common RV myth in a previous article, but we do recommend that you travel with 3 to 5 gallons of fresh, clean water in your tank to help lubricate the tanks, as long as you can dump that water when you get to your destination (or, again, nearby your destination at a facility that had dump capabilities).