1999 Ram Van 3500 Cooling system problem while towing

It's an RV conversion van actually, with only 55K miles. The engine is the 5.9 liter. It has A/C and a good sized trans cooler. Usually, and up till recently, its never had a cooling system problem even when towing a fairly sizeable trailer. Recently, when towing the trailer, even though the temp guage never goes much above midway, it seems to loose coolant. But I don't see a leak. I look at the recovery tank just after stopping and it is almost full even though when I checked it cool, it was only on the line near the bottom of the tank. I think it is running over. Also when the engine does cool down, all the coolant will not return to the radiator, eventually I have to add coolant. I can also hear what sounds like boiling inside the engine block just after I turn off the engine. I've changed the thermostat and the radiator cap, flushed the system and changed the hoses. I'm using a 50:50 mix of standard antifreeze. It seems like overheating, but the temp guage says all is well. Can I increase the antifreeze strength to reduce boiling? What could be causing this?

You have a blown head gasket. A compression, or a leak down test can confirm this.

You have a blown head gasket. A compression, or a leak down test can confirm this.

Thanks for the advice. That thought crossed my mind too. Since this is a van, lack of room alongside the engine makes it really tough to get to certain spark plugs - I managed to change them recently, but not without some knuckle skinning. I don't even know if my compression tester would work for this case. I don't have a leak down tester, but do understand how they work. (Are they $$$ ?) I'll try the comp test as soon as it stops raining (its winter in the Seattle area). We drove it cross country this past fall like this so it can probably wait a little while more. Any tips on accessing the spark plugs or removing the head(s) if it comes to that would be appreciated.

$35.00 if you really want one.
http://www.harborfreight.com/cylinder-leak-down-tester-94190.html

If you have to pull the heads, I would think about pulling the entire engine. It will make the overall job faster and easier.

If you change the plugs, to avoid skinning the knuckles, take the front wheel off on the side you are changing the plugs. Access is really easy then. :)