Before you do that, go over to the Monaco site and do a search for coolant reservoir and pressure caps. Most of those tanks are of a "nylon/plastic" type of material and are subject to rupture if pressures get to high. On my 05 ISL400 in a dynasty, the cap was 8 lbs as I remember. (now sold). In any case, at least give Monaco a call with your coach id number and ask what was original pressure cap supposed to be.
http://www.irv2.com/forums/f115/
2005 Monaco Dynasty Diamond IV 42' ISL 400; Tag Axle; Residential Refrig (sold)
2004 F450, 6.0 auto, 4:30 pumpkin; BrakeSmart; 50 gal aux.tank, 2008 Mobile Suites RE3.