Almost all my vehicles came straight out of the bases with markings intact. I don't see that as an issue.
Very few do make it out. One of the requirements of turn is is that the bumper numbers and other specific unit ID numbers be painted over or ground off.
That being said I have owned two M915's that escaped with numbers intact. In the case of my M927 a little brake cleaner and the black paint that had been sprayed over the numbers came right off the CARC.
I'm not saying that there are official types out there waiting to pounce on you. What I'm saying is that having numbers on a privately owned vehicle that are similar to the numbers on an active duty unit's vehicles might be seen as an attempt to pass your vehicle off as an official military vehicle. This is especially true now that some vehicle series (M900 and FMTV) that are being surplussed out are still in active units.
Most military installations now require anyone that is not an ID holder to get a visitors pass at the gate. They may or may not inspect your vehicle when you enter but they have the right to do so. There are also varying levels of security depending on the nature of the base. I have been to bases to load out or inspect that required me to submit to a background check several days prior to entering. Rolling up to one of those bases in a military vehicle is going to get you looked at very hard while they determine exactly what you are there for and if your paperwork is in order.
If the unit you wish to honor is no longer on the active rolls you are fine. You can check through various sites such as this one:
http://www.armorama.com/modules.php?op=modload&name=Sections&file=index&req=viewarticle&artid=113
If the vehicle is so old that there is no way it could be mistaken for an active vehicle then you are probably fine since it will probably not be painted in todays paint scheme anyway. No one is going to mistake an olive drab M37 for an active duty vehicle. A M923 with desert sand or three color camo is a different story and if you are mistaken for an active duty truck and enter a restricted zone you may be in for some interesting interactions with the powers that be.
I once had a young Second Lt. give me the devil for pumping fuel at the post gas station (not the military vehicle fuel point) into one of my deuces at the local National Guard base. When I showed him my tag and insurance form he calmed down but you get the point.
Honoring someone or some unit is a noble endeavor but brings with it some sense of responsibility.
